Virus

Un article de Vev.

Version du 17 décembre 2007 à 21:56 Pyrococcus (Discuter) ← Différence précédente		Version actuelle Thumperward (Discuter) (tidy headers a bit)
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-	{{Voir homonymes}}	+	{{otheruses}}
-	{{Taxobox virus | Virus | Herpes_simpex_virus.jpg | Virus de l'herpès. }}	+	{{Taxobox
-	<!-- {{Taxobox groupe | {{virus Groupe ??}} }} -->	+	| color = violet
-	{{Taxobox taxons virus |	+	| name = Viruses
-	''Voir [[classification des virus]]''	+	| image = Rotavirus Reconstruction.jpg
-	* {{virus Groupe I}}	+	| image_caption = Rotavirus
-	* {{virus Groupe II}}	+	| virus_group = I–VII
-	* {{virus Groupe III}}	+	| subdivision_ranks = Groups
-	* {{virus Groupe IV}}	+	| subdivision =
-	* {{virus Groupe V}}	+	I: [[dsDNA virus]]es<br />
-	* {{virus Groupe VI}}	+	II: [[ssDNA virus]]es<br />
-	* {{virus Groupe VII}}	+	III: [[dsRNA virus]]es<br />
		+	IV: [[positive-sense ssRNA virus|(+)ssRNA virus]]es<br />
		+	V: [[negative-sense ssRNA virus|(-)ssRNA virus]]es<br />
		+	VI: [[ssRNA-RT virus]]es<br />
		+	VII: [[dsDNA-RT virus]]es
	}}		}}
-	{{Taxobox fin | microbiologie }}	+	{{seealso|Introduction to virus}}
-	Un '''virus''' est une entité [[Biologie|biologique]] qui nécessite une [[cellule (biologie)|cellule]] [[hôte (biologie)|hôte]], dont il utilise les constituants pour se [[reproduction (biologie)|multiplier]]. Les virus existent sous une forme extracellulaire ou intracellulaire. Sous la forme intracellulaire (à l'intérieur de la cellule hôte), les virus sont des éléments génétiques qui peuvent se répliquer de façon indépendante par rapport au [[chromosome]], mais non indépendamment de la cellule hôte. Sous la forme extracellulaire, les virus sont des objets particulaires, infectieux, constitués au minimum d'un [[acide nucléique]] et de [[protéine]]s.	+
-	La '''[[virologie]]''' est la science qui étudie les virus. Elle est étudiée par des ''virologues'' ou des ''virologistes''.	+	A '''virus''' (from the [[Latin]] ''virus'' meaning toxin or poison) is a [[Optical microscope#Limitations of light microscopes|sub-microscopic]] infectious agent that is unable to grow or reproduce outside a [[host (biology)|host]] [[cell (biology)|cell]]. Each viral particle, or virion, consists of genetic material, [[DNA]] or [[RNA]], within a protective protein coat called a [[capsid]]. The capsid shape varies from simple helical and icosahedral (polyhedral or near-spherical) forms, to more complex structures with tails or an [[viral envelope|envelope]]. Viruses infect cellular life forms and are grouped into animal, plant and bacterial types, according to the type of host infected.
-	Le mot ''virus'' est issu du [[latin]] ''virus, i'' (neutre) qui signifie « poison ». Se terminant par un ''s'', il ne prend pas de marque particulière au pluriel en français<ref>Le pluriel latin de ''virus'' n'est pas connu. Bien que se terminant par "-us", on le considère comme un nom neutre de la [[Déclinaisons latines|2<sup>e</sup> déclinaison]]&nbsp;; son pluriel serait alors "vira". La forme "viri" est une faute grammaticale&nbsp;: c'est le pluriel des mots masculins de la 2<sup>e</sup> déclinaison&nbsp;; "virii" est un [[barbarisme]]. [[:en:Plural of virus|En anglais]], la marque du pluriel pour un mot se terminant par "s" est "-es"&nbsp;; la forme "viruses" se retrouve d'ailleurs le plus souvent dans la littérature médicale et professionnelle.</ref>.	+	It has been argued whether viruses are living organisms. Some consider them non-living as they do not meet the criteria of the definition of [[life]]. For example, unlike most organisms, viruses do not have [[Cell (biology)|cells]]. However, viruses have genes and evolve by [[natural selection]]. They have been described as organisms at the edge of life. Viral infections in human as well as animal hosts, usually result in an immune response and disease. Often, a virus is completely eliminated by the [[immune system]]. [[Antibiotic]]s have no effect on viruses, but [[antiviral drug]]s have been developed to treat life-threatening infections. [[Vaccine]]s that produce lifelong [[immunity (medical)|immunity]] can prevent virus infections.
-	==Caractéristiques==	+	== Etymology ==
-	[[Image:Virion.png|thumb|right|Structure de base d'un virus.]]	+
-	Un virus se caractérise par son incapacité à se multiplier par division. Il a besoin pour cela d'utiliser une cellule hôte : un virus est un [[parasite (biologie)|parasite]] intracellulaire obligatoire. Il est composé d'une molécule d'acide nucléique (soit d'[[acide désoxyribonucléique|ADN]] soit d'[[acide ribonucléique|ARN]], simple ou double brin) entourée d'une coque de [[protéine]]s appelée la [[capside]] et parfois d'une enveloppe. Il ne possède en général aucune [[enzyme]] pouvant produire de l'énergie. Les virus sont le plus souvent de très petite taille (comparée à celle d'une [[bactérie]] par exemple), en règle générale inférieure à 250 [[nanomètre]]s ; toutefois, le [[mimivirus]] a une taille de 400 nm, ce qui le rend plus gros que les plus petites bactéries. Ce dernier a aussi la particularité de posséder à la fois de l'ADN et de l'ARN.	+
-	La forme libre du virus (ou particule virale) s'appelle le [[virion]].	+	The word is from the [[Latin]] ''virus'' referring to [[poison]] and other noxious substances, first used in English in 1392.<ref name=Etymology_Dictionary>{{cite web | title = virus | work = The Online Etymology Dictionary | url = http://www.etymonline.com/index.php?term=virus | accessdate = 2007-07-16}}</ref> ''Virulent'', from Latin ''virulentus'' "poisonous" dates to 1400.<ref name=OED>{{cite web | title = virulent, a. | work = The Oxford English Dictionary - Online | url = http://dictionary.oed.com | accessdate = 2007-07-16}}</ref> A meaning of "agent that causes infectious disease" is first recorded in 1728,<ref name=Etymology_Dictionary /> before the discovery of viruses by the [[Russians|Russian]]-[[Ukrainians|Ukrainian]] [[biologist]] [[Dmitry Ivanovsky]] in 1892. The adjective ''viral'' dates to 1948.<ref name=OED2>{{cite web | title = viral, a. | work = The Oxford English Dictionary - Online | url = http://dictionary.oed.com | accessdate = 2007-07-16}}</ref> Today, ''virus'' is used to describe the biological viruses discussed above and as a [[metaphor]] for other parasitically-reproducing things, such as [[meme]]s or [[computer virus]]es (since 1972).<ref name=OED /> The term '''virion''' is also used to refer to a single infective viral particle. The English plural form of ''virus'' is ''viruses''.
-	Il existe une très grande diversité de virus, estimé en [[2007]] à 10<sup>31</sup> qui est bien plus que la diversité des trois règnes ([[bactérie|Bacteria]], [[Archaea]], [[Eucaryote|Eukaryota]]) réunis <ref>''Les virus, ennemis utiles'' (Hors série de ''[[Pour la Science]]''), ''Tant qu’il y aura des virus'' de Ali Saïb</ref>.	+	==History and discovery of viruses==
		+	Viral diseases such as [[rabies]], [[yellow fever]] and [[smallpox]] have affected humans for centuries. There is hieroglyphical evidence of [[polio]] in the ancient Egyptian empire,<ref>Paul GF. (1971) A History of Poliomyelitis. Yale University Press:
		+	New Haven and London.</ref> though the cause of this disease was unknown at the time. In 1717, [[Lady Mary Wortley Montagu|Mary Montagu]], the wife of an English ambassador to the [[Ottoman Empire]], observed local women [[inoculation|inoculating]] their children against [[smallpox]].<ref name=Behbehani_1983>{{cite journal |author=Behbehani AM |title=The smallpox story: life and death of an old disease |journal=Microbiol Rev |volume=47 |issue=4 |pages=455-509 |year=1983 |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=6319980 |pmid=6319980}}</ref> In the late 18th century, [[Edward Jenner]] observed and studied Miss Sarah Nelmes, a milkmaid who had previously caught [[cowpox]] and was found to be immune to [[smallpox]], a similar, but devastating virus. Jenner developed the first [[vaccine]] based on these findings. After lengthy [[vaccination]] campaigns, the [[World Health Organization]] (WHO) certified the eradication of [[smallpox]] in 1979.
-	Tous les êtres vivants peuvent être infectés par des virus. Il existe des virus de bactéries (les [[bactériophage]]s), des virus d'Archaea, des virus d'algues (''[[Phycodnaviridae]]''), des virus de plantes, des virus fongiques, des virus d'invertébrés, des virus de vertébrés chez lesquels on trouve de nombreux agents pathogènes.	+	Viruses themselves were discovered through a combination of new technology and old [[process of elimination]]. In the late 19th century [[Charles Chamberland]] developed a porcelain filter with pores small enough to remove cultured bacteria from their culture medium.<ref name=Horzinek_1997>{{cite journal | author = Horzinek MC| title = The birth of virology | journal = Antonie van Leeuwenhoek | year = 1997 | volume = 71 | pages = 15&ndash;20 | doi=10.1023/A:1000197505492 }}</ref> [[Dimitri Ivanovski]] used this filter to study an infection of tobacco plants, now known as [[tobacco mosaic virus]]. He passed crushed leaf extracts of infected tobacco plants through the filter, then used the filtered extracts to infect other plants, thereby proving that the infectious agent was not a bacterium. Similar experiments were performed by several other researchers, with similar results. These experiments showed that viruses are [[orders of magnitude]]s smaller than bacteria. The term ''virus'' was coined by the Dutch microbiologist [[Martinus Beijerinck]] who showed, using methods based on the work of Ivanovski, that tobacco mosaic disease is caused by something smaller than a bacterium. He developed the term "contagium vivum fluidum" which means “soluble living germ” as first the idea of the virus.<ref>Chung, King-Thom and Ferris, Deam Hunter (1996). Martinus Willem Beijerinck (1851-1931): pioneer of general microbiology. AMS News 62, 539-543. http://www.asm.org/ASM/files/CCLIBRARYFILES/FILENAME/0000000251/621096p539.pdf PDF]</ref>
-	==Découverte==	+	In the early 20th century, [[Frederick Twort]] discovered that bacteria could be infected by viruses.<ref> href="http://encyclopedia.jrank.org/Cambridge/entries/067/Frederick-William-Twort.html">Frederick William Twort</ref> [[Felix d'Herelle]], working independently, showed that a preparation of viruses caused areas of cellular death on thin [[cell culture]]s spread on [[agar]]. Counting the dead areas allowed him to estimate the original number of viruses in the suspension. The invention of [[Electron microscopy]] provided the first look at viruses. In 1935 [[Wendell Stanley]] crystallised the tobacco mosaic virus and found it to be mostly [[protein]].<ref name="pmid17756690">{{cite journal |author=Stanley WM, Loring HS |title=THE ISOLATION OF CRYSTALLINE TOBACCO MOSAIC VIRUS PROTEIN FROM DISEASED TOMATO PLANTS |journal= |volume=83 |issue=2143 |pages=85 |year=1936 |pmid=17756690 |doi=10.1126/science.83.2143.85}}</ref> A short time later the virus was separated into protein and [[nucleic acid]] parts.<ref name="pmid17788438">{{cite journal |author=Stanley WM, Lauffer MA |title=DISINTEGRATION OF TOBACCO MOSAIC VIRUS IN UREA SOLUTIONS |journal= |volume=89 |issue=2311 |pages=345–347 |year=1939 |pmid=17788438 |doi=10.1126/science.89.2311.345}}</ref><ref name="pmid16590772">{{cite journal |author=Tsugita A, Gish DT, Young J, Fraenkel-Conrat H, Knight CA, Stanley WM |title=THE COMPLETE AMINO ACID SEQUENCE OF THE PROTEIN OF TOBACCO MOSAIC VIRUS |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=46 |issue=11 |pages=1463–9 |year=1960 |pmid=16590772 |doi=}}</ref> In 1939, [[Max Delbrück]] and E.L. Ellis demonstrated that, in contrast to cellular organisms, bacteriophage reproduce in "one step", rather than exponentially.<ref name="pmid16791793">{{cite journal |author=Pennazio S |title=The origin of phage virology |journal=Riv. Biol. |volume=99 |issue=1 |pages=103–29 |year=2006 |pmid=16791793 |doi=}}</ref>
-	Les maladies virales comme la [[rage]], la [[fièvre jaune]], la [[variole]], affectent les humains depuis des siècles. Des [[hiéroglyphe]]s mettent en évidence la [[Poliomyélite]] dans l'[[Égypte antique]], les écrits de l’Antiquité [[Grèce antique|gréco]]-[[Rome antique|romaine]] et d’Extrême-Orient décrivent certaines maladies virales. Cependant, la cause de ces maladies est restée inconnue pendant longtemps. À la fin du {{XIXe siècle}}, la conception d’agents infectieux qui n’étaient ni des [[bactérie]]s, ni des [[champignon]]s, ni des [[Parasite (biologie)|parasite]]s était encore difficile.	+
-	Entre [[1887]] et [[1892]], le [[Botanique|botaniste]] [[Russie|russe]] [[Dimitri Ivanovski]] étudia une maladie végétale, la [[virus de la mosaïque du tabac|mosaïque du tabac]], et montra que la [[sève]] des plans malades contenait un agent infectieux qui n’était pas retenue par les [[filtres Chamberland]] conçu par le [[Charles Chamberland|biologiste du même nom]]. Ivanovski pensait qu’il s’agissait d’une [[toxine]] ou bien d’une très petite bactérie. C’est le [[Chimie|chimiste]] [[Pays-Bas|hollandais]] [[Martinus Beijerinck]] qui approfondit ces travaux et écarta l’hypothèse bactérienne, et dénomma le phénomène ''Contagium vivum fluidum''. À la même époque, le virus de la [[fièvre aphteuse]] est le premier virus identifié par [[Friedrich Loeffler]] et [[Paul Frosch]]. Le virus de la fièvre jaune est le premier virus pathogène de l’Homme identifié entre [[1900]] et [[1902]].	+	A major problem for early virologists was the inability to propagate viruses on sterile culture media, as is done with cellular microorganisms. This limitation required medical virologists to infect living animals with infectious material, which is dangerous. The first breakthrough came in 1931, when [[Ernest William Goodpasture]] demonstrated the growth of [[influenza]] and several other viruses in fertile chicken eggs.<ref name="pmid17810781">{{cite journal |author=Goodpasture EW, Woodruff AM, Buddingh GJ |title=THE CULTIVATION OF VACCINE AND OTHER VIRUSES IN THE CHORIOALLANTOIC MEMBRANE OF CHICK EMBRYOS |journal= |volume=74 |issue=1919 |pages=371–372 |year=1931 |pmid=17810781 |doi=10.1126/science.74.1919.371}}</ref> However, some viruses would not grow in chicken eggs, and a more flexible technique was needed for propagation of viruses. The solution came in 1949 when [[John Franklin Enders]], [[Thomas H. Weller]] and [[Frederick Chapman Robbins]] together developed a technique to grow [[polio virus]] in cultures of living animal cells.<ref name="pmid15470207">{{cite journal |author=Rosen FS |title=Isolation of poliovirus--John Enders and the Nobel Prize |journal=N. Engl. J. Med. |volume=351 |issue=15 |pages=1481–3 |year=2004 |pmid=15470207 |doi=10.1056/NEJMp048202}}</ref> Their methods have since been extended and applied to the growth of viruses and other infectious agents that do not grow on sterile culture media.
-	C’est pendant la [[Première Guerre mondiale]] que l’[[Royaume-Uni|anglais]] [[Frederick Twort]] et le [[Microbiologie|microbiologiste]] [[France|franco]]-[[Canada|canadien]] [[Félix d'Hérelle]] mettent en évidence le phénomène de « lyse transmissible » observable par la lyse des bactéries cultivées en milieu solide. Ce phénomène est dû à un virus de bactéries que Félix d'Hérelle baptisa [[bactériophage]]. Les virus des plantes, des animaux, de l’Homme et des bactéries étaient ainsi découverts et leurs listes ne cessèrent de s’allonger au cours du {{XXe siècle}}. L’apparition de la microscopie électronique dans les [[années 1930]] permis l’observation des virus, mais on ne savait toujours pas à cette époque ce qu’ils étaient réellement.	+	==Origins==
		+	The origin of modern viruses is not entirely clear. It may be that no single mechanism can account for their origin.<ref>Holmes EC, Drummond AJ. The evolutionary genetics of viral emergence.Curr Top Microbiol Immunol. 2007;315:51-66.</ref> They do not [[fossil]]ize well, so [[Molecular biology|molecular techniques]] have been the most useful means of hypothesising how they arose.<ref>Liu Y, Nickle DC, Shriner D, Jensen MA, Learn GH Jr, Mittler JE, Mullins JI. Molecular clock-like evolution of human immunodeficiency virus type 1.Virology. 2004 Nov 10;329(1):101-8.</ref> Research in [[microfossil]] identification and molecular biology may yet discern fossil evidence dating to the [[Archean]] or [[Proterozoic]] [[eon (geology)|eons]]. Two main hypotheses currently exist.<ref name="prescott">{{cite book | title=Microbiology| last=Prescott| first=L| date=1993| publisher=Wm. C. Brown Publishers| id=0-697-01372-3}}</ref>
-	Le biochimiste américain [[Wendell Meredith Stanley|Wendell Stanley]] [[Cristallographie|cristallisa]] le virus de la mosaïque du tabac sous forme de [[cristal protéique]] en [[1935]]. L'année suivante des études complémentaires montrèrent que ce cristal contenait également de l’[[ARN]]. Les études ultérieures montrèrent que selon les virus étudiés, ceux-ci étaient composés soit de protéines et d’ARN, soit de protéines et d’ADN. C’est en [[1957]] que [[André Lwoff]] proposa une définition claire et moderne des virus.	+	Small viruses with only a few genes may be runaway stretches of nucleic acid originating from the genome of a living organism. Their genetic material could have been derived from transferable genetic elements such as [[plasmid]]s or [[transposon]]s, that are prone to moving within, leaving, and entering genomes. New viruses are emerging ''de novo'' and therefore, it is not always the case that viruses have "ancestors"<ref>Keese P, Gibbs A. Plant viruses: master explorers of evolutionary space.Curr Opin Genet Dev. 1993 Dec;3(6):873-7.</ref>
-	A partir des [[années 1960]], le développement des [[Culture cellulaire|cultures cellulaires]], de la [[Microscope électronique|microscopie électronique]], puis de la [[biologie moléculaire]] permirent aux scientifiques de progresser dans la compréhension des mécanismes de réplication des virus, dans la réalisation de diagnostics fiables et dans l’élaboration de [[vaccin]].	+	Viruses with larger genomes, such as [[poxvirus]]es, may have once been small cells that parasitised larger host cells. Over time, genes not required by their parasitic lifestyle would have been lost in a streamlining process known as ''retrograde-evolution'' or ''reverse-evolution''. The bacteria [[Rickettsia]] and [[Chlamydia]] are living cells that, like viruses, can only reproduce inside host cells. They lend credence to the streamlining hypothesis, as their parasitic lifestyle is likely to have caused the loss of genes that enabled them to survive outside a host cell.
-	==Origine==	+	It is possible that viruses represent a primitive form of self replicating DNA and are a precursor to life as it is presently defined.<ref>Koonin EV. The Biological Big Bang model for the major transitions in evolution.Biol Direct. 2007 Aug 20;2:21.</ref> Other infectious particles which are even simpler in structure than viruses include [[viroid]]s, [[satellite (biology)|satellite]]s, and [[prion]]s.
-	Il existe plusieurs hypothèses concernant l'origine et l'évolution des virus. Il est probable que tous les virus ne dérivent pas d'un même ancêtre commun et les différents virus peuvent avoir des origines différentes.	+
-	* Les virus et les cellules ont pu apparaître dans la [[soupe primordiale]] en même temps et évoluer parallèlement. Dans ce scénario, au début de l’apparition de la vie, les plus anciens systèmes génétiques d'auto-réplication (probablement de l'[[acide ribonucléique|ARN]]) sont devenus plus complexes et se sont enveloppées dans un sac lipidique pour aboutir au progénote à l'origine des cellules. Une autre forme réplicative aurait pu garder sa simplicité pour former des particules virales.	+
-	* Les virus pourraient dériver de cellules ayant subi une régression. D'après cette hypothèse, les ancêtres des virus auraient été des êtres vivants libres ou des micro-organismes devenus des prédateurs ou des [[Parasite (biologie)|parasite]]s dépendant de leur [[hôte (biologie)|hôte]]. Les relations de [[parasitisme]] entraînent la perte de nombreux [[gène]]s (notamment les gènes pour le [[métabolisme]] apporté par l'hôte). Cet organisme aurait co-évolué avec la cellule hôte et n'aurait conservé que sa capacité à répliquer son [[acide nucléique]] et le mécanisme de transfert de cellule à cellule.	+
-	* Les virus peuvent avoir pour origine des morceaux d'[[acide nucléique|acides nucléiques]] qui se sont « échappés » du [[génome]] cellulaire pour devenir indépendants. Ce phénomène pourrait avoir eu lieu lors d’erreurs au cours de la réplication du matériel génétique. Les virus pourraient aussi avoir pour origine des [[plasmide]]s (molécules d’ADN circulaires) ou des [[transposon]]s (séquence d'ADN capable de se déplacer et de se multiplier dans un [[génome]]).	+
-	== Structure ==	+	==Classification==
		+	{{details|Virus classification}}
		+	In [[taxonomy]], the classification of viruses is difficult owing to the lack of a fossil record and the dispute over whether they are living or non-living.<ref> Rybicki EP (1990) The classification of organisms at the edge of life, or problems with virus systematics. S Aft J Sci 86:182-186</ref><ref name="pmid13481308">{{cite journal |author=LWOFF A |title=The concept of virus |journal=J. Gen. Microbiol. |volume=17 |issue=2 |pages=239–53 |year=1957 |pmid=13481308 |doi=}}</ref> They do not fit easily into any of the [[domain (biology)|domains]] of [[biological classification]] and classification begins at the [[family (biology)|family]] rank. However, the domain name of [[Acytota]] (without cells) has been suggested. This would place viruses on a par with the other domains of [[Eubacteria]], [[Archaea]], and [[Eukarya]]. Not all families are currently classified into orders, nor all genera classified into families.
-	[[Image:Polyomavirus SV40 TEM B82-0338 lores.jpg|thumb|200px|right|Microscopie électronique de plusieurs [[polyomavirus]].]]	+	In 1962 [[André Lwoff]], [[Robert Horne]], and [[Paul Tournier]] were the first to develop a means of virus classification, based on the [[Linnaean taxonomy|Linnaean]] hierarchical system.<ref name="pmid14467544">{{cite journal |author=LWOFF A, HORNE RW, TOURNIER P |title=[A virus system.] |language=French |journal=C. R. Hebd. Seances Acad. Sci. |volume=254 |issue= |pages=4225–7 |year=1962 |pmid=14467544 |doi=}}</ref> This system based classification on [[phylum]], [[class (biology)|class]], [[order (biology)|order]], [[family (biology)|family]], [[genus]], and [[species]]. Viruses were grouped according to their shared properties (not of their hosts) and the type of nucleic acid forming their genomes.<ref name="pmid13931895">{{cite journal |author=LWOFF A, HORNE R, TOURNIER P |title=A system of viruses |journal=Cold Spring Harb. Symp. Quant. Biol. |volume=27 |issue= |pages=51–5 |year=1962 |pmid=13931895 |doi=}}</ref> Following this initial system, a few modifications were made and the [[International Committee on Taxonomy of Viruses]] was developed (ICTV).
-	Une particule virale complète, appelé virion, est composé d’un filament d’[[acide nucléique]], généralement stabilisé par des [[nucléoprotéine]]s basiques, enfermé dans une coque protéique protectrice appelée [[capside]]. La forme de la capside est à la base des différentes morphologies des virus. La taille des virus se situe entre 10 et 400 nm. Les [[génome]]s des virus ne comportent que de quelques gènes à {{formatnum:1200}} gènes. Le plus petit virus connu est le ''[[hépatite D|virus delta]]'' qui parasite lui-même celui de l'[[hépatite B]]. Il ne comporte qu'un seul [[gène]]. Le plus gros virus connu est le ''[[mimivirus]]'' avec un diamètre qui atteint 400 nanomètres et un génome qui comporte {{formatnum:1200}} gènes.	+	====ICTV classification====
		+	The [[International Committee on Taxonomy of Viruses]] (ICTV) developed the current classification system and put in place guidelines that put a greater weighting on certain virus properties to maintain family uniformity. A universal system for classifying viruses, and a unified taxonomy, has been established since 1966. In determining order, taxonomists should consider the type of nucleic acid present, whether the nucleic acid is single- or double-stranded, and the presence or absence of an [[Envelope (biology)|envelope]]. After these three main properties, other characteristics can be considered: the type of host, the capsid shape, immunological properties and the type of disease it causes. The system makes use of a series of ranked [[taxon]]s.
		+	The general structure is as follows:
-	=== Acide nucléique ===	+	:[[Order (biology)|Order]] (''-virales'')
		+	::[[Family (biology)|Family]] (''-viridae'')
		+	:::[[Subfamily]] (''-virinae'')
		+	::::[[Genus]] (''-virus'')
		+	:::::[[Species]] (''-virus'')
-	{{article détaillé|Acide nucléique}}	+	The recognition of orders is very recent; to date, only 3 have been named, most families remain unplaced. The committee does not formally distinguish between subspecies, strains, and isolates. In total there are 3 orders, 56 families, 9 subfamilies, 233 genera. ICTV recognizes about 1,550 virus species but about 30,000 virus strains and isolates are being tracked by virologists.<ref> Virus Taxonomy 8th Reports of the International Committee on Taxonomy of Viruses C.M. Fauquet, M.A. Mayo, J. Maniloff, U. Desselberger, and L.A. Ball (eds)
		+	Academic Press, 1162 pp. (2005) Elsevier Publication Date: 27 May 2005 </ref>
-	Le filament d'acide nucléique peut être de l'[[acide désoxyribonucléique|ADN]] ou de l'[[acide ribonucléique|ARN]]. Il représente le [[génome]] viral. Il peut être circulaire ou linéaire, bicaténaire (double brin) ou monocaténaire (simple brin). Le génome sous forme d'ADN est généralement bicaténaire. Le génome sous forme d'ARN est généralement monocaténaire et peut être à polarité positive (dans le même sens qu'un ARN messager) ou à polarité négative (complémentaire d'un ARN messager). Le peloton central d'acide nucléique est dénommé ''nucléoïde''.	+	The [[Nobel Prize]]-winning biologist [[David Baltimore]] devised the [[Virus classification#Baltimore classification|Baltimore classification]] system.<ref name="pmid4377923">{{cite journal |author=Baltimore D |title=The strategy of RNA viruses |journal=Harvey Lect. |volume=70 Series |issue= |pages=57–74 |year=1974 |pmid=4377923 |doi=}}</ref><ref name="pmid4348509">{{cite journal |author=Temin HM, Baltimore D |title=RNA-directed DNA synthesis and RNA tumor viruses |journal=Adv. Virus Res. |volume=17 |issue= |pages=129–86 |year=1972 |pmid=4348509 |doi=}}</ref> The ICTV classification system is used in conjunction with the Baltimore classification system in modern virus classification.<ref name="pmid15078590">{{cite journal |author=van Regenmortel MH, Mahy BW |title=Emerging issues in virus taxonomy |journal=Emerging Infect. Dis. |volume=10 |issue=1 |pages=8–13 |year=2004 |pmid=15078590 |doi=}}</ref><ref name="pmid10486120">{{cite journal |author=Mayo MA |title=Developments in plant virus taxonomy since the publication of the 6th ICTV Report. International Committee on Taxonomy of Viruses |journal=Arch. Virol. |volume=144 |issue=8 |pages=1659–66 |year=1999 |pmid=10486120 |doi=}}</ref><ref name="pmid15183049">{{cite journal |author=de Villiers EM, Fauquet C, Broker TR, Bernard HU, zur Hausen H |title=Classification of papillomaviruses |journal=Virology |volume=324 |issue=1 |pages=17–27 |year=2004 |pmid=15183049 |doi=10.1016/j.virol.2004.03.033}}</ref>
-	=== Capside ===	+	====Baltimore Classification====
		+	[[Image:Baltimore Classification.png|right|thumb|300px| The Baltimore Classification of viruses is based on the method of viral [[mRNA]] synthesis]]
		+	The [[Virus classification#Baltimore classification|Baltimore classification]] of viruses is based on the mechanism of [[mRNA]] production. Viruses must generate positive strand mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. This classification places viruses into seven groups:
-	{{article détaillé|Capside}}	+	• I: Double-stranded DNA (e.g. [[Adenovirus]]es, [[Herpesvirus]]es, [[Poxvirus]]es)
-	La capside est une coque qui entoure et protège l'acide nucléique viral. Elle est constituée par l'assemblage de structures protéiques. La capside est constituée de sous-unités protéiques appelées protomères. L'ensemble capside et nucléoïde est nommé ''nucléocapside''. La structure de la capside entraîne la forme du virus, ce qui permet de distinguer deux groupes principaux de virus : les virus à ''symétrie cubique'' et les virus à ''symétrie hélicoïdale''.	+	• II: Single-stranded (+)sense DNA (e.g. [[Parvovirus]]es)
-	=== Enveloppe ===	+	• III: Double-stranded RNA (e.g. [[Reovirus]]es)
-	De nombreux virus sont entourés d'une enveloppe (ou ''[[péplos]]'') qui prend naissance au cours de la traversée des membranes cellulaires. Sa constitution est complexe et présente un mélange d'éléments cellulaires et d'éléments d'origine virale. On y trouve des [[protéine]]s, des [[glucide]]s et des [[lipides]]. Les virus possédant une enveloppe sont les ''virus enveloppés''. Les virus ne possédant pas d'enveloppe sont les ''virus nus''.	+	• IV: Single-stranded (+)sense RNA (e.g. [[Picornavirus]]es, [[Togavirus]]es)
-	{| class="wikitable"	+	• V: Single-stranded (-)sense RNA (e.g. [[Orthomyxovirus]]es, [[Rhabdovirus]]es)
-	|bgcolor="#dddddd"|	+
-	|bgcolor="#dddddd"| '''Virus icosaédriques'''	+	• VI: Single-stranded (+)sense RNA with DNA intermediate in life-cycle (e.g. [[Retrovirus]]es)
		+
		+	• VII: Double-stranded DNA with RNA intermediate (e.g. [[Hepadnavirus]]es)
		+
		+	As an example of viral classification, the [[chicken pox]] virus, [[Varicella zoster]] (VZV), belongs to family ''[[Herpesviridae]]'', subfamily ''[[Alphaherpesvirinae]]'' and genus ''[[Varicellovirus]]''. It remains unranked in terms of order. VZV is in Group I of the Baltimore Classification because it is a dsDNA virus that does not use [[reverse transcriptase]].
		+
		+	== Structure ==
		+	A complete virus particle, known as a virion, consists of [[nucleic acid]] surrounded by a protective coat of [[protein]] called a [[capsid]]. Viruses can have a [[lipid]] 'envelope' derived from the host [[cell membrane]]. A capsid is made from proteins encoded by the viral [[genome]] and its shape serves as the basis for [[morphology (biology)|morphological]] and antigenic distinction.<ref name="pmid14019094">{{cite journal |author=CASPAR DL, KLUG A |title=Physical principles in the construction of regular viruses |journal=Cold Spring Harb. Symp. Quant. Biol. |volume=27 |issue= |pages=1–24 |year=1962 |pmid=14019094 |doi=}}</ref><ref name="pmid13309339">{{cite journal |author=CRICK FH, WATSON JD |title=Structure of small viruses |journal=Nature |volume=177 |issue=4506 |pages=473–5 |year=1956 |pmid=13309339 |doi=}}</ref> Virally coded protein subunits will self-assemble to form a capsid, generally requiring the presence of the virus genome. However, complex viruses code for proteins which assist in the construction of their capsid.<ref name=prescott> </ref> Proteins associated with nucleic acid are known as [[nucleoprotein]]s, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid.
		+
		+	In general, there are four main morphological virus types:
		+
		+	{| cellpadding=3 cellspacing=0 border=1 style="border-collapse:collapse"
		+	|colspan=2 bgcolor="#dddddd"| '''Helical viruses'''
		+	|-
		+	| [[Image:Tobacco mosaic virus structure.png|center|thumb|200px|Diagram of a helical capsid]]
		+	| Helical capsids are composed of a single type of subunit stacked around a central axis to form a helical structure which may have a central cavity, or hollow tube. This arrangement results in rod-shaped or filamentous virions: these can be anything from short and highly rigid, to long and very flexible. The genetic material, generally single-stranded RNA, but ssDNA in some cases, is bound into the protein helix, by interactions between the negatively-charged nucleic acid and positive charges on the protein. Overall, the length of a helical capsid is related to the length of the nucleic acid contained within it and the diameter is dependent on the size and arrangement of protomers. The well-studied [[Tobacco mosaic virus]] is an example of a helical virus.
		+	|-
		+	|colspan=2 bgcolor="#dddddd"| '''Icosahedral viruses'''
	|-		|-
-	|[[Image:Coronaviruses 004 lores.jpg|center|thumb|200px|virions icosaédriques au microscope électronique]]	+	| [[Image:Enteric Adenoviruses.jpg|center|thumb|200px|Electron micrograph of icosahedral virions]]
-	|La capside icosaédrique entraîne une apparence sphérique du virus. Les protomères sont organisés en capsomères, disposés de manières régulières et géométriques. Une capsomère, en forme d’anneau, est composée de cinq ou six protomères.<br/>	+	| Icosahedral capsid symmetry results in a spherical appearance of viruses at low magnification but actually consists of capsomers arranged in a regular geometrical pattern, similar to a [[football (soccer)|soccer ball]], hence they are not truly "spherical". Capsomers are ring shaped constructed from five to six copies of protomers. These associate via [[chemical bond|non-covalent bonding]] to enclose the viral nucleic acid, though generally less intimately than helical capsids, and may involve one or more protomers.
-	Parmi les virus icosaédriques, les [[parvovirus]] ont une capside formée de 12 capsomères, les [[poliovirus]] 32 capsomères, les [[papillomavirus]] 72 capsomères tandis que la capside des [[Adenoviridae|adénovirus]] est constituée de 252 capsomères.	+	Icosahedral architecture was employed by [[Buckminster Fuller|R. Buckminster-Fuller]] in his [[geodesic dome]], and is the most efficient way of creating an enclosed robust structure from multiple copies of a single protein.
		+	The number of proteins required to form a spherical virus capsid is denoted by the T-number,<ref name="triang">{{Cite web|url=http://web.archive.org/web/20060223170529/http://rhino.bocklabs.wisc.edu/cgi-bin/virusworld/htdocs.pl?docname=triangulation.html|title=Virus triangulation numbers via Internet Archive|accessdate=2006-04-05}}</ref> where 60&times;''t'' proteins are necessary. In the case of the [[hepatitis B]] virus the T-number is 4, and 240 proteins assemble to form the capsid.
	|-		|-
-	|bgcolor="#dddddd"|	+	|colspan=2 bgcolor="#dddddd"| '''Enveloped viruses '''
-	|bgcolor="#dddddd"| '''Virus hélicoïdaux'''	+
	|-		|-
-	| [[Image:Tobacco mosaic virus structure.png|center|thumb|200px|Shéma d’une capside hélicoïdale]]	+	| [[Image:Chickenpox-virus.jpg|thumb|center|200px|Herpes zoster virus]]
-	|Ces virus sont de longs cylindres (300 à 400 nm), creux, composés d’un type de protomère enroulé en spirale hélicoïdale. Ils peuvent être rigide ou flexible. Le matériel génétique est logé à l’intérieur du tube. Le virus de la [[virus de la mosaïque du tabac|mosaïque du tabac]] est un exemple de virus hélicoïdal très étudié.	+	| Viruses are able to envelope themselves in a modified form of one of the [[cell membranes]] either the outer membrane surrounding an infected host cell, or from internal membranes such as nuclear membrane or endoplasmic reticulum, thus gaining an outer lipid bilayer known as a [[viral envelope]]. This membrane is studded with proteins coded for by the viral genome and host genome; however the lipid membrane itself and any carbohydrates present are entirely host-coded. The Influenza virus and HIV use this strategy.
		+	The viral envelope can give a virion a few distinct advantages over other capsid-only virions, such as protection from enzymes and certain chemicals. The proteins in it can include [[glycoprotein]]s functioning as [[receptor (biochemistry)|receptor molecules]], allowing host cells to recognise and bind these virions, resulting in the possible uptake of the virion into the cell. Most enveloped viruses are dependent on the envelope for infectivity.
	|-		|-
-	|bgcolor="#dddddd"|	+	|colspan=2 bgcolor="#dddddd"| '''Complex viruses'''
-	|bgcolor="#dddddd"| '''Virus enveloppés'''	+
	|-		|-
-	|[[Image:HIV Viron.png|thumb|center|200px|Schéma d’un virus enveloppé : le [[VIH]]]]	+	| [[Image:Tevenphage.png|thumb|center|200px|Diagram of a bacteriophage]]
-	|En plus de la capside, certains virus sont capable de s’entourer d’une structure membranaire emprunté à la cellule hôte. Cette enveloppe membranaire est composé d’une bicouche lipidique qui peut posséder des protéines codées par le génome viral ou le génome de l’hôte. Cette enveloppe donne quelques avantages aux virions par rapport à ceux composés d’une capside seul, comme la protection vis à vis d’enzymes ou de composés chimiques. Les [[glycoprotéine]]s, formant des spicules, fonctionnent comme des [[Récepteur (biochimie)|récepteurs]] permettant de se fixer sur des cellules hôtes spécifiques.<br/>	+	| These viruses possess a capsid which is neither purely helical, nor purely icosahedral, and which may possess extra structures such as protein tails or a complex outer wall. Some [[bacteriophages]] have a complex structure consisting of an icosahedral head bound to a helical tail, the latter of which may have a hexagonal base plate with protruding protein tail fibres.
-	Le [[Myxovirus influenzae|virus de la grippe]] (famille des ''[[Orthomyxoviridae]]''), le virus du [[Syndrome d'immunodéficience acquise|SIDA]] (famille des ''[[Retroviridae]]'') sont des exemples de virus enveloppés.	+	|-
		+	| [[Image:Poxvirus.jpg|thumb|center|200px|Poxvirus]]
		+	|The [[Poxvirus]]es are large, complex viruses which have an unusual [[morphology (biology)|morphology]]. The viral genome is associated with proteins within a central disk structure known as a [[nucleoid]]. The nucleoid is surrounded by a membrane and two lateral bodies of unknown function. The virus has an outer envelope with a thick layer of protein studded over its surface. The whole particle is slightly [[pleiomorphic]], ranging from ovoid to brick shape.<ref>Long GW, Nobel J, Murphy FA, Herrmann KL, and Lourie B (1970) Experience with electron microscopy in the differential diagnosis of smallpox. Applied Microbiology 20(3):497-504.</ref>
		+	|}
		+	===Electron Microscopy===
		+	{{details|Electron Microscopy}}
		+	[[Image:Relative scale.svg|thumb|300px|right|The range of sizes shown by viruses, relative to those of other organisms and [[biomolecule]]s]]
		+	[[Electron Microscopy]] is the commonest method used to study the [[morphology (biology)|morphology]] of viruses. To increase the contrast between viruses and the background, electron-dense "stains" are used. These are solutions of salts of heavy metals such as [[tungsten]], that scatter the electrons from regions covered with the stain. When virus particles are coated with stain (positive staining), fine detail is obscured. [[Negative staining]] overcomes this problem by staining the background only.<ref name="pmid1715774">{{cite journal |author=Kiselev NA, Sherman MB, Tsuprun VL |title=Negative staining of proteins |journal=Electron Microsc. Rev. |volume=3 |issue=1 |pages=43–72 |year=1990 |pmid=1715774 |doi=}}</ref>
		+
		+	=== Size ===
		+	A medium sized virion next to a flea is roughly equivalent to a human next to a mountain twice the size of [[Mount Everest]]. Some [[filovirus]]es have a total length of up to 1400&nbsp;nm, however their capsid diameters are only about 80&nbsp;nm. Most viruses which have been studied have a [[capsid]] diameter between 10 and 300 [[nanometres]]. Most viruses are unable to be seen with a [[light microscope]] but some are as large or larger than the smallest bacteria and can be seen under high optical magnification. More commonly, both scanning and transmission [[electron microscope]]s are used to visualise virus particles.
		+
		+	== Genome ==
		+	{| class = "prettytable" style = "float:right; font-size:85%; margin-left:15px"
		+	|+ Genomic diversity among viruses
		+	! Property || Parameters
	|-		|-
-	|bgcolor="#dddddd"|	+	| ''Nucleic acid'' ||
-	|bgcolor="#dddddd"| '''Virus complexes'''	+	*DNA
		+	*RNA
		+	*Both DNA and RNA
	|-		|-
-	|[[Image:Tevenphage.png|thumb|center|200px|Shéma d’un [[bactériophage]] ]]	+	| ''Shape'' ||
-	|Ces virus possèdent une capside symétrique qui n’est ni hélicoïdale, ni vraiment icosaédrique. Les [[bactériophage]]s comme le [[phage T4]] d’''[[Escherichia coli]]'' sont des virus complexes possédant une tête icosaédrique liée à une queue hélicoïdale à laquelle sont attachés des poils et des fibres caudales.<br/>	+	*Linear
-	Le [[Poxviridae|poxvirus]] est aussi un exemple de virus complexe. C'est le virus animal parmi les plus grands (250 à 350 nm de long sur 200 à 250 nm de large. Certains virus se présentent sous formes bacillaires. C'est le cas du virus de la [[rage]] (famille des ''[[Rhabdoviridae]]'') et du virus [[Ebolavirus|Ebola]].	+	*Circular
		+	*Segmented
		+	|-
		+	| ''Strandedness'' ||
		+	*Single-stranded
		+	*Double-stranded
		+	*Double-stranded with regions of single-strandedness
		+	|-
		+	| ''[[Sense (molecular biology)|Sense]]'' ||
		+	*Positive sense (+)
		+	*Negative sense (-)
		+	*Ambisense (+/-)
	|}		|}
-	==Classification==	+	An enormous variety of genomic structures can be seen among viral species; as a group they contain more structural genomic diversity than the entire kingdoms of either plants, animals, or bacteria.<ref name="flint">{{cite book | title=Principles of Virology| edition=2nd edn |last=Flinth| coauthors=et al.| date=2004| publisher=ASM Press, New York| id=1-55581-259-7}}</ref>
-	{{article détaillé|Classification des virus}}	+	===Nucleic acid===
		+	A virus may employ either [[DNA]] or [[RNA]] as the nucleic acid. Rarely do they contain both, however [[cytomegalovirus]] is an exception to this, possessing a DNA core with several [[mRNA]] segments.<ref name=prescott> </ref> By far most viruses have RNA. Plant viruses tend to have single-stranded RNA and bacteriophages tend to have double-stranded DNA.<ref name="prescott"> </ref> Some virus species possess abnormal [[nucleotide]]s, such as ''hydroxymethylcytosine'' instead of [[cytosine]], as a normal part of their genome.<ref name="prescott"> </ref>
-	Les virus sont classifiés selon la nature de l'[[acide nucléique]] de leur [[génome]] ([[Acide désoxyribonucléique|ADN]] ou [[ARN]]), la structure de l'acide nucléique (monocaténaire ou bicaténaire), la forme de l'acide nucléique (linéaire, circulaire, segmenté ou non). Les données morphologiques peuvent également être prises en compte (présence ou absence d'enveloppe, symétrie de la capside). Souvent, le [[sérogroupage]] est encore utilisé pour raffiner la définition des différences entre virus très proches.	+	===Shape===
		+	Viral genomes may be circular, such as [[polyomavirus]]es, or linear, such as [[adenovirus]]es. The type of nucleic acid is irrelevant to the shape of the genome. Among [[RNA virus]]es, the genome is often divided up into separate parts within the virion and are called ''segmented''. Double-stranded RNA genomes and some single-stranded RNA genomes are segmented.<ref name=prescott> </ref> Each segment often codes for one protein and they are usually found together in one capsid. Every segment is not required to be in the same virion for the overall virus to be infectious, as demonstrated by the [[brome mosaic virus]].<ref name="prescott"> </ref>
-	== Multiplication virale ==	+	===Strandedness===
		+	A viral genome, irrespective of nucleic acid type, may be either single-stranded or double-stranded. Single-stranded genomes consist of an unpaired nucleic acid, analogous to one-half of a ladder split down the middle. Double-stranded genomes consist of 2 complementary paired nucleic acids, analogous to a ladder. Viruses, such as those belonging to the ''[[Hepadnaviridae]]'', contain a genome which is partially double-stranded and partially single-stranded.<ref name="flint"> </ref> Viruses that infect humans include double-stranded RNA (e.g. [[Rotavirus]]), single-stranded RNA (e.g. [[Influenza virus]]), single-stranded DNA (e.g. [[Parvovirus B19]]) and double-stranded DNA ([[Herpesviridae|Herpes virus]]).
-	Les virus ne peuvent se multiplier qu’au sein de cellules vivantes, par réplication de leur acide nucléique. C’est l’interaction du génome viral et de la cellule hôte qui aboutit à la production de nouvelles particules virales. L’infection d’une cellule par un virus, puis la multiplication du virus peuvent se résumer en différentes étapes. Toutefois, après pénétration du virus dans la cellule, ces étapes peuvent différer selon la nature du virus en question et notamment selon qu’il s’agit d’un virus à ADN ou d’un virus à ARN.	+	===Sense===
		+	For viruses with RNA as their nucleic acid, the strands are said to be either [[positive-sense]] (called the plus-strand) or [[negative-sense]] (called the minus-strand) depending on whether it is complementary to viral mRNA. Positive-sense viral RNA is identical to viral mRNA and thus can be immediately [[translation (genetics)|translated]] by the host cell. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an [[RNA polymerase]] before translation. DNA nomenclature is similar to RNA nomenclature, in that the ''coding strand'' for the viral mRNA is complementary to it (-), and the ''non-coding strand'' is a copy of it (+).
-	* '''L'attachement ou adsorption''' : au cours de cette étape, il y a liaison d’une [[protéine]] virale à un récepteur de la surface cellulaire. Les récepteurs eucaryotes peuvent être soit des glycoprotéines, soit des glycosphingolipides. Les récepteurs des [[bactériophage]]s sont des glycoprotéines ou des lipopolysaccharides. Les cellules végétales ne possèdent pas de récepteurs spécifiques aux virus.	+	===Genome size===
-	* '''La pénétration''' : selon les virus, il existe plusieurs mécanismes de pénétration du virus à l’intérieur de la cellule. Chez les [[bactériophage]]s, seul le [[génome]] viral pénètre dans la cellule bactérienne. Chez les virus animaux le virus peut pénétrer par plusieurs mécanismes. Le virus peut pénétrer par [[pinocytose]] : la nucléocapside virale, entourée de la [[membrane plasmique]] pénètre dans la cellule. C’est souvent le cas des virus nus. Dans le cas des virus enveloppés, le virus peut pénétrer soit par fusion (il y a fusion de l’enveloppe virale et de la [[membrane plasmique]] cellulaire) soit par [[endocytose]] (il y a accumulation de particules virales dans des vésicules cytoplasmiques).	+	Genome size in terms of the weight of [[nucleotides]] varies between species. The smallest genomes code for only four proteins and weigh about 10<sup>6</sup> [[Atomic mass unit|Dalton]]s, the largest weigh about 10<sup>8</sup> Daltons and code for over one hundred proteins.<ref name=prescott> </ref> [[RNA virus]]es generally have smaller genome sizes than [[DNA virus]]es due to a higher error-rate when replicating, resulting in a maximum upper size limit. Beyond this limit, errors in the genome when replicating render the virus useless or uncompetitive. To compensate for this, RNA viruses often have segmented genomes where the genome is split into smaller molecules, thus reducing the chance of error.<ref>Pressing J, Reanney DC. Divided genomes and intrinsic noise.J Mol Evol. 1984;20(2):135-46.</ref> In contrast, DNA viruses generally have larger genomes due to the high fidelity of their replication enzymes.<ref name="flint"> </ref>
-	* '''La décapsidation''' après la pénétration (ou en même temps), il y a libération de l’[[acide nucléique]]. Selon les virus, la décapsidation peut avoir lieu dans le [[cytoplasme]] ou dans le [[noyau (biologie)|noyau]].	+
-	* '''La réplication ou multiplication virale''' : lors de cette phase, il y a réplication du génome, expression du génome sous forme d’[[ARNm]] ([[Transcription (biologie)|transcription]]) et [[traduction]] des ARNm en [[protéine]]s par la machinerie cellulaire. Selon les types de virus et la nature de leur [[génome]], le mécanisme de la multiplication virale peut être très différent.	+
-	* '''L'assemblage''' (phase de maturation) : il y a assemblage et maturation des virus dans les cellules infectées. Il y a encapsidation du [[génome]]. Les virus enveloppés acquièrent leur enveloppe par bourgeonnement, au détriment de la [[membrane plasmique]] ou de la [[Enveloppe nucléaire|membrane nucléaire]] de la cellule-hôte.	+
-	* '''La libération''' : le virus reconstitué est libéré à l’extérieur de la cellule.	+
-	==Culture des virus==	+	===Gene reassortment===
-	[[Image:Plaque assay macro.jpg|thumb|left|Culture de virus : technique des plages de lyse.]]	+	There is an evolutionary advantage in having a segmented genome. Different strains of a virus with a segmented genome, from a pig or a bird or a human for example, such as [[Influenza virus]], can shuffle and combine with other genes producing progeny viruses or (offspring) that have unique characteristics. This is called reassortment or ''viral sex''.<ref> Goudsmit, Jaap. Viral Sex. Oxford Univ Press, 1998.ISBN-13: 9780195124965 ISBN-10: 0195124960</ref> This is one reason why Influenza virus constantly changes.<ref> Zhou NN, Senne DA, Landgraf JS, Swenson SL, Erickson G, Rossow K, Liu L, Yoon K, Krauss S, Webster RG. Genetic reassortment of avian, swine, and human influenza A viruses in American pigs.J Virol. 1999 Oct;73(10):8851-6.</ref>
-	Afin de mieux connaître leur biologie, leur multiplication, leur cycle de reproduction et éventuellement afin de préparer des [[vaccin]]s, il est nécessaire de cultiver les virus. Ceux-ci peuvent se multiplier uniquement au sein de cellules vivantes. Les virus infectant les cellules eucaryotes sont cultivées sur des [[culture cellulaire|cultures de cellules]] obtenues à partir de tissus animaux ou végétaux. Les cellules sont cultivées dans un récipient en verre ou en plastique, puis sont infectées par le virus étudié. Les virus animaux peuvent aussi être cultivés sur œufs embryonnés et parfois chez l’animal, lorsque la culture in vitro est impossible. Les virus bactériens peuvent également être cultivés par inoculation d’une culture bactérienne sensible. Les virus de végétaux peuvent aussi être cultivés sur des monocouches de tissus végétaux, des suspensions cellulaires ou sur des plantes entières.	+
-	Les virus peuvent ensuite être quantifiés de différentes manières. Ils peuvent être comptés directement grâce à la microscopie électronique. Dans le cas des virus bactériens, la technique des plaques (ou plages) est très utilisée pour évaluer le nombre de virus dans une suspension. Une dilution de suspension virale est ajouté à une suspension bactérienne, puis l’ensemble est réparti dans des [[boîte de Petri|boîtes de Petri]]. Après culture, des zones claires (plages) à la surface de la gélose sont la conséquence de la destruction d’une bactérie et des bactéries adjacentes par un virion.	+	===Genetic recombination===
		+	[[Genetic recombination]] is the process by which a strand of DNA is broken and then joined to the end of a different DNA molecule. This can occur when viruses infect cells simultaneously and studies of viral evolution have shown that recombination has been rampant in the species studied.<ref name="pmid10573145">{{cite journal |author=Worobey M, Holmes EC |title=Evolutionary aspects of recombination in RNA viruses |journal=J. Gen. Virol. |volume=80 ( Pt 10) |issue= |pages=2535–43 |year=1999 |pmid=10573145 |doi=}}</ref> Recombination is common to both RNA and DNA viruses.<ref name="pmid15578739">{{cite journal |author=Lukashev AN |title=Role of recombination in evolution of enteroviruses |journal=Rev. Med. Virol. |volume=15 |issue=3 |pages=157–67 |year=2005 |pmid=15578739 |doi=10.1002/rmv.457}}</ref><ref name="pmid10479778">{{cite journal |author=Umene K |title=Mechanism and application of genetic recombination in herpesviruses |journal=Rev. Med. Virol. |volume=9 |issue=3 |pages=171–82 |year=1999 |pmid=10479778 |doi=}}</ref>
-	Les virus peuvent être purifiés grâce à diverses méthodes de biochimie ([[centrifugation]] différentielle, précipitation, dénaturation, digestion enzymatique).	+	===Genetic change===
		+	Viruses undergo genetic change by several mechanisms. These include a process called [[genetic drift]] where individual bases in the DNA or RNA [[mutate]] to other bases. Most of these [[point mutations]] are silent in that they do not change the protein that the gene encodes, but others can confer evolutionary advantages such as resistance to [[antiviral drugs]].<ref>Pan XP, Li LJ, Du WB, Li MW, Cao HC, Sheng JF. Differences of YMDD mutational patterns, precore/core promoter mutations, serum HBV DNA levels in lamivudine-resistant hepatitis B genotypes B and C. J Viral Hepat. 2007 Nov;14(11):767-74.</ref> [[Antigenic shift]] where there is a major change in the [[genome]] of the virus. This occurs as a result of [[Genetic recombination|recombination]] or [[reassortment]] (see above). When this happens with [[influenza]] viruses, [[pandemics]] may result.<ref>Hampson AW, Mackenzie JS. The influenza viruses.Med J Aust. 2006 Nov 20;185(10 Suppl):S39-43.</ref><ref>Nakajima K. The mechanism of antigenic shift and drift of human influenza virus Nippon Rinsho. 2003 Nov;61(11):1897-903.</ref> By genome rearrangement where the structure of the gene changes although no mutations have necessarily occurred.<ref>Hundley F, McIntyre M, Clark B, Beards G, Wood D, Chrystie I, Desselberger U. Heterogeneity of genome rearrangements in rotaviruses isolated from a chronically infected immunodeficient child.J Virol. 1987 Nov;61(11):3365-72.</ref>
-	==Débat sur son statut de forme de vie==	+	RNA viruses are much more likely to mutate than DNA viruses for the reasons outlined above. Viruses often exist as [[quasispecies]] or swarms of viruses of the same species but with slightly different genome nucleoside sequences. Such quasispecies are a prime target for [[natural selection]].<ref>Metzner KJ. Detection and significance of minority quasispecies of drug-resistant HIV-1. HIV Ther. 2006 Dec;11(4):74-81.</ref>
-	Les virus possèdent des constituants en commun avec les [[organisme vivant|cellules vivantes]], comme un acide nucléique ([[Acide désoxyribonucléique|ADN]] ou [[ARN]]) et des [[protéine]]s. Cependant, selon la définition du biochimiste [[Wendell Meredith Stanley|Wendell Stanley]], les virus sont de « simples » associations de molécules biologiques. Ils sont le fruit d’une auto-organisation de molécules organiques et ne sont donc pas vivants. [[François Jacob]] insiste aussi sur cette caractéristique des virus : « placés en suspension dans un milieu de culture, ils ne peuvent ni [[métabolisme|métaboliser]], ni produire ou utiliser de l’énergie, ni croître, ni se multiplier, toutes fonctions communes aux êtres vivants »<ref>François Jacob, Qu’est-ce que la vie ? ''in'' La Vie, Université de tous les savoirs, Editions Odile Jacob, 2002.</ref>. Les virus ne peuvent se multiplier qu’en utilisant l’équipement enzymatique d’une cellule vivante. De plus, les virus contiennent bien un acide nucléique, de l’ADN ou de l’ARN mais jamais les deux, à la différence des cellules vivantes.	+
-	Néanmoins, au cours des dernières années, la description de nouveaux virus relance le débat sur le caractère vivant ou non vivant des virus. Le [[Mimivirus]], infectant une [[amibe]], possède dans son [[génome]] {{formatnum:1200}} gènes, soit plus que certaines [[bactérie]]s. De plus certains de ces gènes participeraient à la synthèse protéique et à des mécanismes de réparation de l’ADN <ref>Raoult D, Audic S, Robert C, Abergel C, Renesto P, Ogata H, La Scola B, Suzan M, Claverie JM. The 1.2-megabase genome sequence of Mimivirus. ''Science.'' 2004 Nov 19;306(5700):1344-50.</ref>. Il existe chez le mimivirus une trentaine de gènes présents habituellement chez les organismes cellulaires mais absents chez les virus. Par ailleurs, le virus ATV d’[[Archaea|archéobactéries]] présente lui aussi des caractéristiques étonnantes. Ce virus en forme de citron présente la particularité de se modifier en dehors du contexte cellulaire par un mécanisme actif. Il est capable de s’allonger à chaque extrémité à une température de 80°C, température à laquelle vit son hôte ''[[Acidianus]]'' à proximité des [[source hydrothermale|sources hydrothermales]]<ref>M. Haring ''et al.'', Independant virus development outside a host, ''Nature'', vol. 436, pp. 1101-1102, 2005</ref>.	+	==Replication==
		+	Viral populations do not grow through [[cell division]], because they are acellular; instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves. A virus can still cause degenerative effects within a cell without causing its death; collectively these are termed [[cytopathic effect]]s.
		+	===Virus life cycle===
		+	The life cycle of viruses differs greatly between species (see below) but there are six ''basic'' stages in the life cycle of viruses:
		+	[[Image:Virus Replication.svg|right|thumb|250px|A virus attaches to the host cell and enters endocytosis. The capsid protein dissociates and the viral RNA is transported to the nucleus. In the nucleus, the viral polymerase complexes transcribe and replicate the RNA. Viral mRNAs migrate to cytoplasm where they are translated into protein. Then the newly synthesized virions [[viral shedding|bud]] from infected cell.]]
		+	*'''Attachment''' is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. This specificity determines the host range of a virus. For example, the human immunodeficiency virus ([[HIV]]) infects only human [[T cells]], because its surface protein, [[gp120]], can interact with [[CD4]] and receptors on the T cell's surface. This mechanism has evolved to favour those viruses that only infect cells that they are capable of replicating in. Attachment to the receptor can induce the viral-envelope protein to undergo changes that results in the fusion of viral and cellular membranes.
		+	*'''Penetration''': following attachment, viruses enter the host cell through receptor mediated [[endocytosis]] or membrane fusion.
		+	*'''Uncoating''' is a process that viral [[capsid]] is removed is degraded by viral [[enzymes]] or host enzymes thus releasing the viral genomic nucleic acid.
		+	*'''Replication''' involves synthesis of viral messenger RNA ([[mRNA]]) for viruses except positive sense RNA viruses (see above), viral [[Protein biosynthesis|protein synthesis]] and assembly of viral proteins and viral genome replication.
		+	*Following the '''assembly''' of the virus particles post-translational modification of the viral proteins often occurs. In viruses such as [[HIV]], this modification, (sometimes called maturation), occurs ''after'' the virus has been released from the host cell.<ref name="pmid11451488">{{cite journal |author=Barman S, Ali A, Hui EK, Adhikary L, Nayak DP |title=Transport of viral proteins to the apical membranes and interaction of matrix protein with glycoproteins in the assembly of influenza viruses |journal=Virus Res. |volume=77 |issue=1 |pages=61–9 |year=2001 |pmid=11451488 |doi=}}</ref>
		+	*Viruses are '''released''' from the host cell by lysis (see below) . Enveloped viruses (e.g., HIV) typically are released from the host cell by “[[viral shedding|budding]]”. During this process, the virus acquires its phospholipid envelope which contains embedded viral glycoproteins.
-	Les virus ont aussi un rôle dans l’évolution. Patrick Forterre propose même l’hypothèse que les virus seraient les « inventeurs » de l’ADN <ref>Patrick Forterre, Three RNA cells for ribosomal lineages and three DNA viruses to replicate their genomes : a hypothesis for the origin of cellular domain, ''PNAS'', vol. 103 (10), pp. 3669-3674, 2006</ref>. À l’[[origine de la vie]], l’ARN dominait (hypothèse du monde à ARN) et assurait à la fois les fonctions de stockage et transmission de l’information génétique et de catalyse des réactions chimiques. L’ADN serait apparu ensuite et sélectionné en raison de sa plus grande stabilité. D’après Patrick Forterre le premier organisme à ADN serait un virus. L'ADN conférerait au virus le pouvoir de résister à des [[enzyme]]s dégradant les [[génome]]s à ARN, arme de défense probable des protocellules. On retrouve le même principe chez des virus actuels, qui altèrent leur ADN pour résister à des enzymes produites par des bactéries infectées.	+	====DNA viruses====
		+	Animal [[DNA virus]]es, such as [[herpesvirus]]es, enter the host via [[endocytosis]], the process by which cells take in material from the external environment. Frequently after a chance collision with an appropriate surface receptor on a cell, the virus penetrates the cell, the viral genome is released from the capsid and host polymerases begin transcribing viral mRNA. New virions are assembled and released either by cell lysis or by [[viral shedding|budding]] off the cell membrane.
-	Le débat entre le caractère vivant ou inerte des virus est encore aujourd’hui ouvert. Répondre à cette question en amène une autre : qu’est-ce que la vie ? D’après Ali Saïb, {{citation|la notion du vivant est une notion dynamique, évoluant en fonction de nos connaissances. En conséquence, la frontière entre la matière inerte et le vivant est tout aussi instable}} <ref>Ali Saïb, Les virus, inertes ou vivants ? ''in'' Pour la Science, décembre 2006.</ref>.	+	====RNA viruses====
		+	Animal [[RNA viruses]] can be placed into about four different groups depending on their modes of replication. The [[Sense (molecular biology)|polarity]] of the RNA largely determines the replicative mechanism, as well as whether the genetic material is single-stranded or double-stranded. Some [[RNA virus]]es are actually DNA based but use an RNA-intermediate to replicate. RNA viruses are dependent on virally encoded [[RNA replicase]] to create copies of their genomes.
-	==Virus des procaryotes==	+	====Reverse transcribing viruses====
-	[[Image:Bacteriophage.jpg|thumb|right|Bactériophages au microscope électronique.]]	+	[[Reverse transcribing viruses]] replicate using reverse transcription, which is the formation of DNA from an RNA template. Viruses containing RNA genomes use a DNA intermediate to replicate, whereas those containing DNA genomes use an RNA intermediate during genome replication. Both types use the [[reverse transcriptase]] enzyme to carry out the nucleic acid conversion. both types are susceptible to [[antiviral drug]]s that inhibit the reverse transcriptase enzyme, e.g. [[zidovudine]] and [[lamivudine]].
-	Il existe deux catégories de virus de [[procaryote]]s selon le type d’hôte qu’ils parasitent. La première catégorie regroupe ceux qui infectent les bactéries et sont appelés [[bactériophage]]s. La deuxième catégorie regroupe ceux qui infectent les [[Archaea|archaeobactéries]].	+
-	Il existe quatre grands groupes morphologiques de virus de procaryotes.	+
-	* Les virus à symétrie binaire. Ce groupe représente près de 96% des virus de procaryotes et correspond aux familles des ''Myoviridae'', des ''Siphoviridae'' et des ''Podoviridae''.	+
-	* Les virus à symétrie cubique avec une capside icosaédrique mais pas de queue comme les ''Microviridae''.	+
-	* Les virus à symétrie hélicoïdale qui ont une forme de filaments comme les ''Inoviridae'' comme le phage [[M13 (virologie)|M13]].	+
-	* Les virus pléomorphes, sans capsides véritable mais possédant une enveloppe. Ce groupe rassemble six familles de virus dont cinq regroupent des virus infectant seulement les archéobactéries. Certains virus d’archéobactéries sont pléomorphes, alors que d’autres ont des formes de bouteilles, de citron, de fuseau<ref>Prangishvili, D., P. Forterre, and R. A. Garrett. 2006. Viruses of the Archaea: a unifying view. Nat Rev Microbiol 4:837-48.</ref>.	+
-	Les bactériophages possèdent un rôle dans les écosystèmes. Par exemple, dans les écosystèmes aquatiques, ils participent au contrôle de l’abondance et de la diversité bactérienne. <ref>K. E. Wommack and R. R. Colwell (2000) Virioplankton: Viruses in Aquatic Ecosystems. Microbiol. Mol. Biol. Rev. 64, 69-114</ref>	+	An example of the first type is [[HIV]] which is a [[retrovirus]]. Retroviruses often integrate the DNA produced by [[reverse transcription]] into the host genome. This is why HIV infection can at present, only be treated and not cured.
-	{{clr}}	+
-	==Virus des plantes==	+	Examples of the second type are the [[Hepadnaviridae]] which includes [[Hepatitis B]] virus and the [[Caulimoviridae]] - e.g. [[Cauliflower mosaic virus]].
-	[[Image:TMV.jpg|thumb|Microscopie électronique de particules du virus de la mosaïque du tabac.]]	+	====Bacteriophages====
		+	{{Main|Bacteriophage}}
		+	[[Image:Phage.jpg|thumb|150px|right|Transmission electron micrograph of multiple [[bacteriophage]]s attached to a bacterial cell wall]]
		+	[[Bacteriophage]]s infect specific bacteria by binding to [[receptor (biochemistry)|surface receptor molecule]]s and then enter the cell. Within a short amount of time, in some cases, just minutes, bacterial [[polymerase]] starts translating viral mRNA into protein. These proteins go on to become either new virions within the cell, helper proteins which help assembly of new virions, or proteins involved in cell [[lysis]]. Viral enzymes aid in the breakdown of the cell membrane, and in the case of the [[T4 phage]], in just over twenty minutes after injection over three hundred phages could be released.
-	La structure des virus des [[plante]]s ou [[phytovirus]], est similaire à celle des virus bactériens et animaux. Beaucoup de virus végétaux se présentent sous la forme de minces et longues hélices. La majorité ont un génome composé d’[[ARN]].	+	==Lifeform debate==
-	Les virus de végétaux peuvent être disséminés par le vent ou par des vecteurs comme les [[insecte]]s et les [[nématode]]s, parfois par les graines et le [[pollen]]. Les virus peuvent aussi contaminer la plante par l’intermédiaire d’une blessure ou d’une [[Greffe (botanique)|greffe]].	+	Viruses have been described as ''organisms at the edge of life''<ref>Rybicki ''ibid''</ref> but argument continues over whether viruses are truly alive. According to the [[United States Code]], they are considered [[microorganism]]s in the sense of [[biological weaponry]] and malicious use. Scientists, however, are divided. Things become more complicated as they look at [[viroids]] and [[prions]]. Viruses resemble other organisms in that they possess genes, and can evolve–in infected cells–by [[natural selection]].<ref>Holmes EC.PLoS Biol. 2007 Oct 2;5(10):e278. Viral Evolution in the Genomic Age</ref><ref>Shackelton LA, Holmes EC.Phylogenetic evidence for the rapid evolution of human B19 erythrovirus.J Virol. 2006 Apr;80(7):3666-9.</ref>
-	Différents types de symptômes peuvent apparaître sur la plante infectée. Les virus peuvent provoquer des taches ou des flétrissements sur les feuilles et les fleurs. Des tumeurs peuvent survenir sur les tiges ou les feuilles.<br/>	+	They can reproduce by creating multiple copies of themselves through self-assembly.
-	Le virus de la [[virus de la mosaïque du tabac|mosaïque du tabac]] (TMV ou tobamovirus) est un exemple très étudié de virus de végétaux.	+
-	{{clr}}	+	Viruses do not have a [[cell (biology)|cell]] structure (regarded as the basic unit of life), although they do have genes. Additionally, although they reproduce, they do not self-metabolize and require a host cell to replicate and synthesise new products. However, bacterial species such as [[Rickettsia]] and [[Chlamydia]], are considered living organisms, but are unable to reproduce outside a host cell.
-	==Virus des insectes==	+	An argument can be made that accepted forms of life use [[cell division]] to reproduce, whereas viruses spontaneously assemble within cells. The comparison is drawn between viral self-assembly and the autonomous growth of non-living [[crystallization|crystal]]s. Virus self-assembly within host cells has implications for the study of the [[origin of life]], as it lends credence to the hypothesis that life could have started as self-assembling organic molecules.<ref>Vlassov, Alexander V. (Jul 2005). "The RNA World on Ice: A New Scenario for the Emergence of RNA Information". Journal of Molecular Evolution 61: 264-273. </ref>
-	Les [[baculovirus]] sont des virus d’[[insecte]]s très étudiés. Ils infectent principalement les [[lépidoptère]]s. La larve de l’insecte s’infecte en ingérant de la nourriture. À partir du tube digestif, l’infection peut se transmettre aux autres tissus.	+	If viruses are considered alive, then the criteria specifying life will have to exclude the cell. If viruses are said to be alive, the question could follow of whether even smaller infectious particles, such as [[viroid]]s and [[prion]]s, are alive.
-	L'utilisation de virus pathogènes d'invertébrés dans la lutte contre les [[insecte ravageur|insectes ravageurs]] des cultures et des forêts pourraient être l'un des moyens pour limiter ou remplacer les insecticides chimiques.<br/>	+
-	Les baculovirus sont aussi utilisés en [[biologie moléculaire]] pour exprimer un gène étranger (protéine recombinante) dans des cultures de cellules d'insecte.<br/>	+
-	Par ailleurs, certains virus de végétaux sont transmis par des invertébrés mais ne se multiplient pas chez ces vecteurs.	+
-	==Virus des champignons==	+	==Viruses and disease==
		+	:''For more examples of diseases caused by viruses see [[List of infectious diseases]]
		+	Examples of common human diseases caused by viruses include the [[common cold]], [[influenza|the flu]], [[chickenpox]] and [[cold sores]]. Serious diseases such as [[Ebola]], [[AIDS]], [[avian influenza]] and [[SARS]] are caused by viruses. The relative ability of viruses to cause disease is described in terms of [[virulence]]. Other diseases are under investigation as to whether they too have a virus as the causative agent, such as the possible connection between [[Human Herpesvirus Six]] (HHV6) and neurological diseases such as [[multiple sclerosis]] and [[chronic fatigue syndrome]]. There is current controversy over whether the [[borna virus]], previously thought of as causing [[neurology|neurological]] disease in horses, could be responsible for [[psychiatry|psychiatric]] illness in humans.<ref name=Chen_1999>{{cite journal |author=Chen C, Chiu Y, Wei F, Koong F, Liu H, Shaw C, Hwu H, Hsiao K |title=High seroprevalence of Borna virus infection in schizophrenic patients, family members and mental health workers in Taiwan |journal=Mol Psychiatry |volume=4 |issue=1 |pages=33-8 |year=1999 |pmid=10089006}}</ref>
-	Les virus des [[champignon]]s sont particuliers car ils se propagent lors de la fusion cellulaire. Il n'y a pas de virions extracellulaires. Chez les [[levure]]s comme ''[[Saccharomyces]]'', les virus sont transmis au moment du brassage cytoplasmique lors de la fusion cellulaire. Les champignons filamenteux comme ''[[Penicillium]]'' ou le champignon de Paris ''[[Agaricus bisporus]]'' peuvent également être infectés par des virus. Cela peut entraîner des problèmes lors de production.	+	Viruses have different mechanisms by which they produce disease in an organism, which largely depends on the species. Mechanisms at the cellular level primarily include cell [[lysis]], the breaking open and subsequent death of the cell. In [[multicellular organism]]s, if enough cells die the whole organism will start to suffer the effects. Although viruses cause disruption of healthy [[homeostasis]], resulting in disease, they may exist relatively harmlessly within an organism. An example would include the ability of the [[herpes simplex virus]], which cause [[coldsore]]s, to remain in a dormant state within the human body. This is called latency<ref>Margolis TP, Elfman FL, Leib D, Pakpour N, Apakupakul K, Imai Y, Voytek C. Spontaneous reactivation of herpes simplex virus type 1 in latently infected
		+	murine sensory Ganglia.J Virol. 2007 Oct;81(20):11069-74. Epub 2007 Aug 8.</ref> and is a characteristic of the [[herpes viruses]] including [[Epstein-Barr virus]] which causes glandular fever and [[Varicella zoster]] virus which causes [[chicken pox]]. Latent chickenpox infections return in later life as the disease called [[disease|shingles]].
-	==Virus et maladie chez l'Homme==	+	Some viruses can cause life-long or [[Chronic (medical)|chronic]] infections where the viruses continue to replicate in the body despite the hosts' defense mechanisms.<ref name="pmid17931183">{{cite journal |author=Bertoletti A, Gehring A |title=Immune response and tolerance during chronic hepatitis B virus infection |journal=Hepatol. Res. |volume=37 Suppl 3 |issue= |pages=S331–8 |year=2007 |pmid=17931183 |doi=10.1111/j.1872-034X.2007.00221.x}}</ref> This is common in [[Hepatitis B virus]] and [[Hepatitis C Virus]] infections. People chronically infected with Hepatitis B virus are known as carriers who serve as reservoirs of infectious virus. In some populations, with a high proportion of carriers, the disease is said to be [[Endemic (epidemiology)|endemic]].<ref name="pmid17645465">{{cite journal |author=Nguyen VT, McLaws ML, Dore GJ |title=Highly endemic hepatitis B infection in rural Vietnam |journal= |volume= |issue= |pages= |year=2007 |pmid=17645465 |doi=10.1111/j.1440-1746.2007.05010.x}}</ref> When diagnosing Hepatitis B virus infections it is important to distinguish between [[Acute (medical)|acute]] and [[Chronic (medical)|chronic]] infections.<ref name="pmid17664817">{{cite journal |author=Rodrigues C, Deshmukh M, Jacob T, Nukala R, Menon S, Mehta A |title=Significance of HBV DNA by PCR over serological markers of HBV in acute and chronic patients |journal=Indian journal of medical microbiology |volume=19 |issue=3 |pages=141–4 |year=2001 |pmid=17664817 |doi=}}</ref>
		+	===Epidemiology===
		+	Viral [[epidemiology]] is the branch of medical science dealing with the transmission and control of virus infections in humans. Transmission of viruses can be “vertical”, that is from mother to child, or “horizontal” which means from person to person. Examples of [[vertical transmission]] include [[Hepatitis B virus]] and [[HIV]] where the baby is born already infected with the virus.<ref name="pmid17825648">{{cite journal |author=Fowler MG, Lampe MA, Jamieson DJ, Kourtis AP, Rogers MF |title=Reducing the risk of mother-to-child human immunodeficiency virus transmission: past successes, current progress and challenges, and future directions |journal=Am. J. Obstet. Gynecol. |volume=197 |issue=3 Suppl |pages=S3–9 |year=2007 |pmid=17825648 |doi=10.1016/j.ajog.2007.06.048}}</ref> Another, more rare, example is [[Varicella zoster virus]] which although causing relatively mild infections in humans can be fatal to the foetus and newly born baby.<ref name="pmid11190597">{{cite journal |author=Sauerbrei A, Wutzler P |title=The congenital varicella syndrome |journal=Journal of perinatology : official journal of the California Perinatal Association |volume=20 |issue=8 Pt 1 |pages=548–54 |year=2000 |pmid=11190597 |doi=}}</ref>
		+	[[Horizontal transmission]] is the commonest mechanism of spread of viruses in populations. Transmission can be exchange of blood, by sexual activity e.g. [[HIV]] and [[Hepatitis B]] and [[Hepatitis C]], by mouth by exchange of [[saliva]], e.g. [[Epstein-Barr virus]] or from contaminated food or water e.g. [[Norovirus]], by breathing in viruses in the form of [[aerosol]]s e.g. [[Influenza virus]] and by insect vectors such as mosquitoes e.g. [[dengue]].
		+	The rate or speed of transmission of virus infections depends on factors that include [[population density]], the number of susceptible individuals, (i.e. those who are not immune),<ref> Garnett GP. Role of herd immunity in determining the effect of vaccines against sexually transmitted disease.J Infect Dis. 2005 Feb 1;191 Suppl 1:S97-106.</ref> the quality of health care and the weather.<ref name="pmid16544901">{{cite journal |author=Platonov AE |title=[The influence of weather conditions on the epidemiology of vector-borne diseases by the example of West Nile fever in Russia] |language=Russian |journal=Vestn. Akad. Med. Nauk SSSR |volume= |issue=2 |pages=25–9 |year=2006 |pmid=16544901 |doi=}}</ref>
-	[[Image:Ebola Virus TEM PHIL 1832 lores.jpg|thumb|Virus [[Ebola (virus)|Ebola]].]]	+	=== Epidemics and pandemics ===
		+	{{details|List of epidemics}}
		+	[[Image:Reconstructed Spanish Flu Virus.jpg|thumb|250px|right|The reconstructed [[1918 influenza]] virus]]
		+	[[Indigenous peoples of the Americas|Native American]] populations were devastated by contagious diseases, particularly [[smallpox]], brought to the Americas by European colonists. It is unclear how many Native Americans were killed by foreign diseases after the arrival of Columbus in the Americas, but the numbers have been estimated to be close to 70% of the indigenous population. The damage done by this disease may have significantly aided European attempts to displace or conquer the native population.<ref>Ranlet P. The British, the Indians, and smallpox: what actually happened at Fort Pitt in 1763? Pa Hist. 2000;67(3):427-41.</ref><ref>Van Rijn K. "Lo! The poor Indian!" colonial responses to the 1862-63 smallpox epidemic inBritish Columbia and Vancouver Island.Can Bull Med Hist. 2006;23(2):541-60.</ref><ref>Patterson KB, Runge T. Smallpox and the Native American.Am J Med Sci. 2002 Apr;323(4):216-22.</ref><ref>Sessa R, Palagiano C, Scifoni MG, di Pietro M, Del Piano M.
		+	The major epidemic infections: a gift from the Old World to the New?
		+	Panminerva Med. 1999 Mar;41(1):78-84.</ref><ref>Bianchine PJ, Russo TA.
		+	The role of epidemic infectious diseases in the discovery of America.
		+	Allergy Proc. 1992 Sep-Oct;13(5):225-32.</ref><ref>Hauptman LM.
		+	Smallpox and American Indian; Depopulation in Colonial New York.
		+	N Y State J Med. 1979 Nov;79(12):1945-9.</ref><ref>Fortuine R.
		+	Smallpox decimates the Tlingit (1787).
		+	Alaska Med. 1988 May-Jun;30(3):109.</ref>
-	Le [[rhume]], la [[grippe]], la [[varicelle]], la [[rougeole]], la [[mononucléose infectieuse]] sont des exemples de maladies humaines virales relativement courantes. Des maladies plus sévères comme le [[Syndrome d'immunodéficience acquise|SIDA]], le [[SRAS]], la [[grippe aviaire]], la [[variole]] sont aussi causées par des virus. Le virus [[Ebola (virus)|Ebola]] entraîne des [[Fièvre hémorragique virale|fièvres hémorragiques]]. La capacité d’un virus d’entraîner une maladie est décrite en terme de [[virulence]].	+	{{main|Spanish flu}}
-	Les virus possèdent différentes stratégies, différents mécanismes grâce auxquels ils peuvent produire des maladies. Le virus pénètre dans une cellule hôte spécifique et prend le contrôle de ses fonctions normales. Au niveau cellulaire, les effets cytopathogènes des virus peuvent entraîner divers effets néfastes. Les capacités de synthèse des [[protéine]]s des cellules infectées peuvent être inhibées, la [[chromatine]] est fragmenté par des enzymes virales. Des particules virales s’accumulent dans le [[cytoplasme]] ou le noyau des cellules infectées. Les virus peuvent ensuite provoquer la lyse et la mort des cellules hôtes. La lyse des cellules entraîne la libération des particules virales et permet la dissémination du virus.	+	A [[pandemic]] is a world-wide epidemic. The 1918 flu pandemic, commonly referred to as the [[Spanish flu]], was a [[Pandemic Severity Index|category 5]] influenza pandemic caused by an unusually severe and deadly [[Influenza A virus]]. The victims were often healthy young adults, in contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or otherwise weakened patients.
		+	<br />The [[Spanish flu]] pandemic lasted from 1918 to 1919. Older estimates say it killed 40–50 million people<ref name=Patterson1>{{cite journal | last =Patterson | first = KD | coauthors = Pyle GF | title=The geography and mortality of the 1918 influenza pandemic. | journal= Bull Hist Med. | year=1991 | month=Spring | volume=65 | issue=1 | pages = 4–21 | id = PMID 2021692}}</ref>
-	Lorsque le virus pénètre dans une cellule non permissive, il ne peut pas se multiplier. Son génome peut cependant subsister sous la forme d’un [[épisome]] libre ou intégré au génome cellulaire. Il y a transformation cellulaire virale lorsque le génome du virus entre en interaction avec l’ADN du génome cellulaire. On appelle ces virus des [[virus oncogène]]s. Parmi ceux-ci, les [[rétrovirus]], en s’intégrant dans le génome cellulaire, peuvent devenir [[tumorigène]] et possèdent donc la capacité d’entraîner des [[cancer]]s.	+	{{main|Aids}}
-	La classification des principaux groupes de virus et leur correspondance en pathologie, se trouve dans l'encyclopédie médicale Vulgaris. Elle est basée sur le type de molécule d'acides nucléiques (ARN ou ADN) qui constitue la particule virale ou virion.[http://www.vulgaris-medical.com/encyclopedie/virus-4811.html]	+	[[Image:Ebola Virus TEM PHIL 1832 lores.jpg|thumb|250px|right|The [[Ebola]] virus]]
		+	[[Acquired Immune Deficiency Syndrome]]<br /> Most researchers believe that HIV originated in [[sub-Saharan Africa]] during the [[twentieth century]];<ref name=Gao>
-	==Prévention et traitement==	+	{{ cite journal
		+	| author=Gao, F., Bailes, E., Robertson, D. L., Chen, Y., Rodenburg, C. M., Michael, S. F., Cummins, L. B., Arthur, L. O., Peeters, M., Shaw, G. M., Sharp, P. M. and Hahn, B. H.
		+	| title=Origin of HIV-1 in the Chimpanzee Pan troglodytes troglodytes
		+	| journal=Nature
		+	| year=1999
		+	| pages=436&ndash;441
		+	| volume=397
		+	| issue=6718
		+	| id={{PMID |9989410}} {{doi|10.1038/17130}}
-	[[Image:Polio EM PHIL 1875 lores.PNG|thumb|Le virus de la [[polio]].]]	+	}}</ref> it is now a [[pandemic]], with an estimated 38.6 [[million]] people now living with the disease worldwide.<ref name=UNAIDS2006>{{
-	Étant donné que les virus utilisent la machinerie cellulaire de l’hôte pour se reproduire à l’intérieur même de la cellule, il est difficile de les éliminer sans tuer la cellule hôte. L’approche médicale la plus efficace est la [[vaccination]] qui permet de résister à l’infection. Divers médicaments permettent de traiter les [[symptôme]]s liés à l’infection. Les patients demandent souvent à leurs [[Médecine générale|médecins]] qu’ils leurs prescrivent des [[antibiotique]]s, mais ils sont sans effet sur les virus. Les antibiotiques interfèrent en effet avec des constituants ou le métabolisme des [[bactérie]]s et permettent donc de traiter seulement les maladies d’origine bactérienne.<br/>	+	cite book
-	Si les virus sont considérés comme des particules non vivantes en dehors du contexte cellulaire, ils ne peuvent pas être « tués » mais sont inactivés. Divers méthodes de [[désinfectant|désinfection]] in vitro permettent d’inactiver les virus ([[hypochlorite de sodium]] à 1 %, [[éthanol]] à 70 %, [[glutaraldéhyde]] à 2 %, [[formaldéhyde]]).	+	| author =[[UNAIDS]]
		+	| year = 2006
		+	| title = 2006 Report on the global AIDS epidemic
		+	| chapter = Overview of the global AIDS epidemic
		+	| chapterurl = http://data.unaids.org/pub/GlobalReport/2006/2006_GR_CH02_en.pdf
		+	| accessdate = 2006-06-08
		+	| format= PDF
		+	}}</ref> As of January 2006, the [[Joint United Nations Programme on HIV/AIDS]] (UNAIDS) and the [[World Health Organization]] (WHO) estimate that AIDS has killed more than 25 million people since it was first recognized on [[June 5]], [[1981]], making it one of the most destructive [[epidemic]]s in [[recorded history]].<ref> Mawar N, Saha S, Pandit A, Mahajan U.
		+	The third phase of HIV pandemic: social consequences of HIV/AIDS stigma &
		+	discrimination & future needs.Indian J Med Res. 2005 Dec;122(6):471-84. Review.</ref>
		+	[[Image:Marburg virions TEM 275 lores.jpg|thumb|250px|right|The [[Marburg virus]]]]
-	==Application==	+	{{main|Ebola}}
-	Les virus sont importants pour l’étude de la [[biologie moléculaire]] et la [[biologie cellulaire]], car ils fournissent des systèmes simples qui peuvent être utilisés pour la manipulation et la compréhension des fonctions cellulaires. Par exemple, les virus présentent en général un matériel génétique simplifié et aident à la compréhension des mécanismes moléculaires de la [[génétique]] comme la [[Réplication|réplication de l’ADN]], la [[Transcription (biologie)|transcription]], les modifications post-transcriptionnels de l’[[ARN]], la [[traduction (biologie)|traduction]], le transport des [[protéine]]s et l’[[immunologie]].	+	Several highly lethal viral pathogens are members of the [[Filoviridae]]. Filoviruses are filament-like viruses that cause [[viral hemorrhagic fever]], and include the [[Ebola]] and [[Marburg virus]]es. The Marburg virus attracted widespread press attention in April 2005 for an outbreak in [[Angola]]. Beginning in October 2004 and continuing into 2005, the outbreak was the world's worst epidemic of any kind of viral hemorrhagic fever.<ref>Towner JS, Khristova ML, Sealy TK, Vincent MJ, Erickson BR, Bawiec DA, HartmanAL, Comer JA, Zaki SR, Stroher U, Gomes da Silva F, del Castillo F, Rollin PE,Ksiazek TG, Nichol ST.
		+	Marburgvirus genomics and association with a large hemorrhagic fever outbreak in
		+	Angola.J Virol. 2006 Jul;80(13):6497-516.</ref>
-	Les virus peuvent de plus être utilisés comme [[vecteur (biologie)|vecteur]] pour introduire un gène dans une cellule. Cet outil est utilisé par exemple pour permettre à la cellule de produire une protéine recombinante ou pour étudier l’effet de l’introduction du nouveau gène dans le [[génome]].<br/>	+	=== Viruses and cancer ===
-	Certains virus sont utilisés en [[thérapie génique]] comme vecteur, pour soigner diverses maladies. Dans certaines maladies génétiques, un gène défectueux provoque les symptômes. Les virus vecteur permettraient de cibler des cellules spécifiques et remplacer le gène en question par un gène normal. <br/>	+	{{details|Virus cancer link}}
-	Les virus sont également utilisés dans la lutte contre le cancer. Certains virus sont capables de détruire spécifiquement des cellules cancéreuses.	+	[[Image:Leukemia cells that contain Epstein Barrvirus using a FA staining technique PHIL 2984 lores.jpg|right|thumb|250px|Human leukaemia cells infected by [[Epstein Barr virus]]]]
		+	Viruses are an established cause of [[malignancy]] in humans and other species.
		+	The main viruses associated with human cancers are [[human papillomavirus]], [[hepatitis B]] and [[hepatitis C]] virus, [[Epstein-Barr virus]], and [[human T-lymphotropic virus]].
		+	Hepatitis viruses, including [[hepatitis B]] and [[hepatitis C]], can induce a [[Chronic (medical)|chronic]] viral infection that leads to [[liver cancer]].<ref> {{cite journal |author=Koike K |title=Hepatitis C virus contributes to hepatocarcinogenesis by modulating metabolic and intracellular signalling pathways |journal=J. Gastroenterol. Hepatol. |volume=22 Suppl 1 |issue= |pages=S108–11 |year=2007 |pmid=17567457 |doi=10.1111/j.1440-1746.2006.04669.x}}</ref><ref> {{cite journal |author=Hu J, Ludgate L |title=HIV-HBV and HIV-HCV coinfection and liver cancer development |journal=Cancer Treat. Res. |volume=133 |issue= |pages=241–52 |year=2007 |pmid=17672044 |doi=}} </ref> Infection by [[Human T-lymphotropic virus]] can lead to Tropical Spastic Paraparesis and Adult T-cell leukaemia<ref> {{cite journal |author=Bellon M, Nicot C |title=Telomerase: a crucial player in HTLV-I-induced human T-cell leukemia |journal=Cancer genomics & proteomics |volume=4 |issue=1 |pages=21–5 |year=2007 |pmid=17726237 |doi=}}</ref> [[Human papillomaviruses]] are an established cause of cancers of [[cervix]], skin, [[anus]], and [[penis]].<ref> {{cite journal |author=Schiffman M, Castle PE, Jeronimo J, Rodriguez AC, Wacholder S |title=Human papillomavirus and cervical cancer |journal=Lancet |volume=370 |issue=9590 |pages=890–907 |year=2007 |pmid=17826171 |doi=10.1016/S0140-6736(07)61416-0}}</ref> Within the [[Herpesviridae]], Kaposi’s sarcoma-associated herpesvirus causes [[Kaposi’s sarcoma]] and Body cavity lymphoma and Epstein–Barr virus causes Burkitt’s lymphoma, Hodgkin’s lymphoma, B lymphoproliferative disease and [[Nasopharyngeal carcinoma]].<ref> {{cite journal |author=Klein E, Kis LL, Klein G |title=Epstein-Barr virus infection in humans: from harmless to life endangering virus-lymphocyte interactions |journal=Oncogene |volume=26 |issue=9 |pages=1297–305 |year=2007 |pmid=17322915 |doi=10.1038/sj.onc.1210240}}</ref>
-	== Exemples de virus pathogènes pour l'Homme ==	+	===Laboratory diagnosis===
-	{{colonnes|taille = 35|	+	[[Image:CPE rounding.jpg|right|thumb|250px|Cells infected with [[Herpes simplex virus]]. The rounding of the cells their detachment from the cell sheet is the typical cytopathic effect produced by this virus]]
-	* [[VIH]], virus du [[Syndrome d'immunodéficience acquise|SIDA]]	+	In the diagnostic laboratory virus infections are confirmed by several methods that include:
-	* [[Retroviridae|Rétrovirus]]	+
-	* [[Coxackie A virus]]	+
-	* [[Ebola (virus)|Ebola]]	+
-	* Virus de la [[variole]]	+
-	* Virus de la [[grippe]]	+
-	* Virus de la [[fièvre jaune]]	+
-	* [[Virus du Nil occidental]]	+
-	* [[Cytomégalovirus]]	+
-	* [[Rotavirus]]	+
-	* Virus de l'[[Hépatite C]]	+
-	* [[Virus simien 40]] ou SV40	+
-	}}	+
-	== Notes et références ==	+	*Growth of the virus in a [[cell culture]] from a specimen taken from the patient.
-	{{colonnes|taille = 35|<references />}}	+
-	==Voir aussi==	+	*Detection of virus-specific [[IgM]] antibody (see below) in the blood.
-	{{Autres projets|	+
-	commons=Category:Viruses|	+
-	wikispecies=Virus|	+
-	wikt=Virus| <!--Wiktionnaire-->	+
-	v=| <!--Wikiversité-->	+
-	b=| <!--Wikilivre-->	+
-	s=| <!--Wikisource-->	+
-	q=| <!--Wikiquote-->	+
-	n=Des scientifiques ont recréé le virus de la grippe espagnole de 1918| <!--Wikinews-->	+
-	}}	+
-	=== Articles connexes ===	+	*Detection of virus antigens by [[ELISA]] in tissues and fluids.
-	* [[Biosûreté]].	+
-	* [[Maladie infectieuse]].	+
-	* [[Classification des virus]].	+
-	* [[Virus informatique]].	+
-	* [[Phytovirus]].	+
-	=== Liens externes ===	+	*Detection of virus encoded DNA and RNA by [[PCR]].
-	* {{fr}} [http://edumed.unige.ch/apprentissage/module1/introduction/apprentissage/virologyBasic/index.html Introduction à la virologie] : document élaboré par Laurent Roux (et réalisé par Daniel Scherly) de la faculté de médecine de l'université de Genève (Suisse). Révision du 14/10/2005.	+
-	* {{fr}} [http://vvanuxem.free.fr/html/generalites/generalites_viro.html Généralités en virologie] : notes de cours proposées par deux étudiants de la faculté de La Rochelle (France). Version du 12/03/2003.	+
-	* {{fr}} [http://anne.decoster.free.fr/d1viro/vgclass.html Principes actuels de classification des virus] : extraits de cours de microbiologie, par Anne Decoster et al., de la faculté libre de médecine de Lille (France). Version du 22/05/2002.	+
-	<!-- 200710290039 : lien non valide. -->	+
-	<!--* {{fr}} [http://lyon-sud.univ-lyon1.fr/bacterio-viro/DESLYON/Fiches/chapitre2/index.html Les principales familles de virus pathogènes pour les humains].-->	+
-	=== Bibliographie ===	+	*Observation of virus particles by [[electron microscopy]].
-	* Pierre Ardoin : ''Virus et diagnostic virologique''. Paris, Maloine Éditeur, 1983, 997p.	+
-	* Thierry Borrel : ''Les Virus. Diversité et organisation du monde viral. Interactions avec le vivant''. Nathan Université, Paris, 1996.	+
-	* Gessain A., Manuquerra J.C. : ''Les virus émergents''. Collection « Que sais-je ? », Presses Universitaires de France, 2006.	+
-	* Madigan, M. T., Martinko, J. M. : ''Brock Biology of Microorganisms, 11th Ed''. Pearson Prentice Hall, Upper Saddle River, NJ, 2005.	+
-	* Perry J., Staley J., Lory S. : ''Microbiologie''. Éditions Dunod, 2004.	+
-	* Prescott, L.M., Harley, J.P. Klein, D.A. : ''Microbiologie 2{{e}} édition''. DeBoeck eds, 2003.	+
-	{{Multi bandeau|Portail microbiologie|portail biologie}}	+	=== Prevention and treatment ===
		+	Because viruses use the machinery of a host cell to reproduce and reside within them, they are difficult to eliminate without killing the host cell. The most effective [[medicine|medical]] approaches to viral diseases so far are [[vaccination]]s to provide resistance to infection, and [[antiviral drugs]] which treat the symptoms of viral infections.
-	{{Lien BA|en}}	+	===Host immune response===
-	{{Lien AdQ|ja}}	+	The body's first line of defense against viruses is the [[innate immune system]]. This comprises cells and other mechanisms that defend the host from infection in a non-specific manner. This means that the cells of the innate system recognize, and respond to, pathogens in a generic way, but unlike the [[adaptive immune system]], it does not confer long-lasting or protective immunity to the host.<ref name=Alberts>{{cite book | last = Alberts| first = Bruce| coauthors = Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walters | title = Molecular Biology of the Cell; Fourth Edition | publisher = Garland Science| date = 2002 | location = New York and London | url = http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=mboc4.TOC&depth=2 | id = ISBN 0-8153-3218-1}}</ref>
-	[[Catégorie:Virus|*]]	+	[[RNA interference]] is an important innate defense against viruses.<ref>Ding SW, Voinnet O. Antiviral immunity directed by small RNAs. Cell. 2007 Aug 10;130(3):413-26.</ref> Many viruses have a replication strategy that involves double-stranded RNA [dsRNA]. When such a virus infects a cell, it releases its RNA molecule or molecules, which immediately bind to a protein complex called [[Dicer]] that cuts the RNA into smaller pieces. A biochemical pathway called the RISC complex is activated which degrades the viral [[mRNA]] and the cell survives the infection. Rotaviruses avoid this mechanism by not uncoating fully inside the cell and by releasing newly produced mRNA through pores in the particles inner capsid. The genomic [[dsRNA]] remains protected inside the core of the virion.<ref name="pmid15579070">{{cite journal |author=Patton JT, Vasquez-Del Carpio R, Spencer E |title=Replication and transcription of the rotavirus genome |journal=Curr. Pharm. Des. |volume=10 |issue=30 |pages=3769–77 |year=2004 |pmid=15579070 |doi=}}</ref><ref name="pmid15010218">{{cite journal |author=Jayaram H, Estes MK, Prasad BV |title=Emerging themes in rotavirus cell entry, genome organization, transcription and replication |journal=Virus Res. |volume=101 |issue=1 |pages=67–81 |year=2004 |pmid=15010218 |doi=10.1016/j.virusres.2003.12.007}}</ref>
		+
		+	When the [[adaptive immune system]] of a [[vertebrate]] encounters a virus, it produces specific [[antibodies]] which bind to the virus and render it non-infectious. This is called [[humoral immunity]]. Two types of antibodies are important. The first called [[IgM]] is highly effective at neutralizing viruses but is only produced by the cells of the immune system for a few weeks. The second, called, [[IgG]] is produced indefinitely. The presence of IgM in the blood of the host is used to test for acute infection, whereas IgG indicates an infection sometime in the past.<ref>Greer S, Alexander GJ. Viral serology and detection.
		+	Baillieres Clin Gastroenterol. 1995 Dec;9(4):689-721</ref> Both types of antibodies are measured when tests for [[Immunity (medical)|immunity]] are carried out.<ref>Laurence JC. Hepatitis A and B immunizations of individuals infected with humanimmunodeficiency virus.Am J Med. 2005 Oct;118 Suppl 10A:75S-83S.</ref>
		+
		+	A second defense of vertebrates against viruses is called [[cell-mediated immunity]] and involves immune cells known as [[T cells]]. The body's cells constantly display short fragments of their proteins on the cell's surface, and if a T cell recognizes a suspicious viral fragment there, the host cell is destroyed by ''T killer'' cells and the virus-specific T-cells proliferate. Cells such as the [[macrophage]] are specialists at this [[antigen presentation]].<ref>Cascalho M, Platt JL. Novel functions of B cells.Crit Rev Immunol. 2007;27(2):141-51.</ref><ref>Khatri M, Sharma JM. Modulation of macrophages by infectious bursal disease virus.Cytogenet Genome Res. 2007;117(1-4):388-93</ref>
		+
		+	Not all virus infections produce a protective immune response in this way. HIV evades the immune system by constantly changing the amino acid sequence of the proteins on the surface of the virion. These persistent viruses evade immune control by sequestration, blockade of [[antigen presentation]], [[cytokine]] resistance, evasion of [[natural killer cell]] activities, escape from [[apoptosis]], and [[antigenic shift]].<ref>Hilleman MR. Strategies and mechanisms for host and pathogen survival in acute and persistent viral infections.Proc Natl Acad Sci U S A. 2004 Oct 5;101 Suppl 2:14560-6. Epub 2004 Aug 5.</ref>
		+
		+	The production of [[interferon]] is an important host defense mechanism.<ref>Le Page C, Genin P, Baines MG, Hiscott J. Interferon activation and innate immunity.Rev Immunogenet. 2000;2(3):374-86.</ref>
		+
		+	===Vaccines===
		+	{{details|Vaccination}}
		+	[[Vaccination]] is a cheap and effective way of preventing infections by viruses. Vaccines were used to prevent viral infections long before the discovery of the actual viruses. Their use has resulted in a dramatic decline in morbidity (illness) and mortality (death) associated with viral infections such as [[polio]], [[measles]], [[mumps]] and [[rubella]].<ref name="pmid17068034">{{cite journal |author=Asaria P, MacMahon E |title=Measles in the United Kingdom: can we eradicate it by 2010? |journal=BMJ |volume=333 |issue=7574 |pages=890–5 |year=2006 |pmid=17068034 |doi=10.1136/bmj.38989.445845.7C}}</ref> [[Smallpox]] infections have been eradicated.<ref name="pmid16989262">{{cite journal |author=Lane JM |title=Mass vaccination and surveillance/containment in the eradication of smallpox |journal=Curr. Top. Microbiol. Immunol. |volume=304 |issue= |pages=17–29 |year=2006 |pmid=16989262 |doi=}}</ref> Currently vaccines are available to prevent over thirteen viral infections of humans<ref name="pmid16364754">{{cite journal |author=Arvin AM, Greenberg HB |title=New viral vaccines |journal=Virology |volume=344 |issue=1 |pages=240–9 |year=2006 |pmid=16364754 |doi=10.1016/j.virol.2005.09.057}}</ref> and more are used to prevent viral infections of animals.<ref name="pmid17892169">{{cite journal |author=Pastoret PP, Schudel AA, Lombard M |title=Conclusions--future trends in veterinary vaccinology |journal=Rev. - Off. Int. Epizoot. |volume=26 |issue=2 |pages=489–94, 495–501, 503–9 |year=2007 |pmid=17892169 |doi=}}</ref> Vaccines can consist of live or killed viruses.<ref name="pmid16494719">{{cite journal |author=Palese P |title=Making better influenza virus vaccines? |journal=Emerging Infect. Dis. |volume=12 |issue=1 |pages=61–5 |year=2006 |pmid=16494719 |doi=}}</ref> Live vaccines contain weakened forms of the virus that causes the disease. Such viruses are called attenuated. Live vaccines can be dangerous when given to people with a weak immunity, (who are described as [[immunocompromised]]), because in these people the weakened virus can cause the original disease.<ref name="pmid1090805">{{cite journal |author=Thomssen R |title=Live attenuated versus killed virus vaccines |journal=Monographs in allergy |volume=9 |issue= |pages=155–76 |year=1975 |pmid=1090805 |doi=}}</ref> Biotechnology and genetic engineering techniques are used to produce subunit vaccines. These vaccines use only the [[capsid]] proteins of the virus. [[Hepatitis B]] vaccine is an example of this type of vaccine.<ref name="pmid3018891">{{cite journal |author=McLean AA |title=Development of vaccines against hepatitis A and hepatitis B |journal=Rev. Infect. Dis. |volume=8 |issue=4 |pages=591–8 |year=1986 |pmid=3018891 |doi=}}</ref> Subunit vaccines are safe for [[immunocompromised]] patients because they cannot cause the disease.<ref name="pmid16221073">{{cite journal |author=Casswall TH, Fischler B |title=Vaccination of the immunocompromised child |journal=Expert review of vaccines |volume=4 |issue=5 |pages=725–38 |year=2005 |pmid=16221073 |doi=10.1586/14760584.4.5.725}}</ref>
		+
		+	===Antiviral drugs===
		+	{{details|Antiviral drug}}
		+	[[Image:DT chemical structure.png|left|thumb|100px|The true DNA base [[thymidine]]]]
		+	[[Image:Zidovudine.svg|right|thumb|100px|The antiviral drug [[Zidovudine]] - [[AZT]]]]
		+
		+	Over the past twenty years the development of [[antiviral drug]]s has increased rapidly. This has been driven by the AIDS epidemic. Antiviral drugs are often [[nucleoside analogues]], (fake DNA building blocks), which viruses incorporate into their genomes during replication. The life-cycle of the virus is then halted because the newly synthesised DNA is inactive. This is because these analogues lack the [[hydroxyl groups]] which along with [[phosphorus]] atoms, link together to form the strong "backbone" of the DNA molecule. This is called DNA [[chain termination]].<ref name="pmid15592828">{{cite journal |author=Magden J, Kääriäinen L, Ahola T |title=Inhibitors of virus replication: recent developments and prospects |journal=Appl. Microbiol. Biotechnol. |volume=66 |issue=6 |pages=612–21 |year=2005 |pmid=15592828 |doi=10.1007/s00253-004-1783-3}}</ref> Examples of nucleoside analogues are [[aciclovir]] for [[Herpesviridae|Herpes virus]] infections and [[lamivudine]] for [[HIV]] and [[Hepatitis B]] virus infections. [[Aciclovir]], is one of the oldest and most frequently prescribed antiviral drugs.<ref name="pmid6355051">{{cite journal |author=Mindel A, Sutherland S |title=Genital herpes - the disease and its treatment including intravenous acyclovir |journal=J. Antimicrob. Chemother. |volume=12 Suppl B |issue= |pages=51–9 |year=1983 |pmid=6355051 |doi=}}</ref>
		+	[[Image:G chemical structure.png|left|thumb|100px|[[Guanosine]]]][[Image:Aciclovir.svg|right|thumb|100px|The guanosine analogue [[Aciclovir]]]] Other antiviral drugs in use target different stages of the viral life cycle. [[HIV]] is dependent on a proteolytic enzyme called the [[HIV-1 protease]] for it to become fully infectious. There is a class of drugs called [[protease inhibitors]] which have been designed to inactivate the enzyme.
		+
		+	[[Hepatitis C]] is caused by an [[RNA]] virus. In 80% of people infected the disease is [[Chronic (medical)|chronic]] and without treatment they are [[infected]] and [[infectious]] for the remainder of their lives. However, there is now an effective treatment using the nucleoside analogue drug [[ribavirin]] combined with [[interferon]]<ref>Witthoft T, Moller B, Wiedmann KH, Mauss S, Link R, Lohmeyer J, Lafrenz M,Gelbmann CM, Huppe D, Niederau C, Alshuth U. Safety, tolerability and efficacy of peginterferon alpha-2a and ribavirin in chronic hepatitis C in clinical practice: The German Open Safety Trial. J Viral Hepat. 2007 Nov;14(11):788-96.</ref> The treatment of chronic [[Asymptomatic carrier|carrier]]s of the [[Hepatitis B]] virus by using a similar strategy using [[lamivudine]] is being developed.<ref>Rudin D, Shah SM, Kiss A, Wetz RV, Sottile VM. Interferon and lamivudine vs. interferon for hepatitis B e antigen-positive hepatitis B treatment: meta-analysis of randomized controlled trials.Liver Int. 2007 Nov;27(9):1185-93.</ref>
		+
		+	==Applications==
		+	===Life sciences and Medicine===
		+	Viruses are important to the study of [[molecular biology|molecular]] and [[cellular biology]] as they provide simple systems that can be used to manipulate and investigate the functions of cells. The study and use of viruses have provided valuable information about aspects of cell biology. For example, viruses been useful in the study of [[genetics]] and helped our understanding of the basic mechanisms of [[molecular genetics]], such as [[DNA replication]], [[transcription (genetics)|transcription]], [[RNA processing]], [[translation (genetics)|translation]], [[protein]] transport, and [[immunology]].
		+
		+	[[Image:Gene therapy.jpg|right|thumb|250px|[[Gene therapy]] using an [[Adenovirus]] vector]]
		+
		+	[[genetics|Geneticists]] often use viruses as [[vector (biology)|vectors]] to introduce genes into cells that they are studying. This is useful for making the cell produce a foreign substance, or to study the effect of introducing a new gene into the genome. In similar fashion, [[virotherapy]] uses viruses as vectors to treat various diseases, as they can specifically target cells and DNA. It shows promising use in the treatment of cancer and in [[gene therapy]].
		+
		+	Eastern European scientists have used [[phage therapy]] as an alternative to antibiotics for some time and interest in this approach is increasing, due to the high level of [[antibiotic resistance]] now found in some pathogenic bacteria.<ref name="pmid16258815">{{cite journal |author=Matsuzaki S, Rashel M, Uchiyama J, ''et al'' |title=Bacteriophage therapy: a revitalized therapy against bacterial infectious diseases |journal=J. Infect. Chemother. |volume=11 |issue=5 |pages=211-9 |year=2005 |pmid=16258815 |doi=10.1007/s10156-005-0408-9}}</ref>
		+
		+	===Materials science and nanotechnology===
		+	Current trends in nanotechnology promise to make much more versatile use of viruses. From the viewpoint of a materials scientist, viruses can be regarded as organic nanoparticles.<ref> Proceedings of SPIE -- Volume 6413Smart Materials IV, Nicolas H. Voelcker, Editor, 64130F (Dec. 22, 2006). Hybrid organic-inorganic nanoparticles: controlled incorporation of gold nanoparticles into virus-like particles and application in surface-enhanced Raman spectroscopy
		+	Marcus Niebert, James Riches, Mark Howes, Charles Ferguson, Robert G. Parton, Anton P. J. Middelberg, Llew Rintoul, and Peter M. Fredericks.Queensland Univ. of Technology (Australia)
		+	(published online Dec. 22, 2006)</ref>
		+	Their surface carries specific tools designed to cross the barriers of their host cells. The size and shape of viruses, and the number and nature of the functional groups on their surface, is precisely defined. As such, viruses are commonly used in materials science as scaffolds for covalently linked surface modifications. A particular quality of viruses is that they can be tailored by directed evolution. The powerful techniques developed by life sciences are becoming the basis of engineering approaches towards nanomaterials, opening a wide range of applications far beyond biology and medicine.<ref name=fischlechner>{{cite journal |author=Fischlechner M, Donath E |title=Viruses as Building Blocks for Materials and Devices |url=http://dx.doi.org/doi:10.1002/anie.200603445 |journal=Angewandte Chemie International Edition |volume= |issue= |pages= |year=2007 |doi=10.1002/anie.200603445}}</ref>
		+
		+	In April 2006 scientists at the [[Massachusetts Institute of Technology]] (MIT) created [[nanotechnology|nanoscale]] metallic wires using a [[Genetic engineering|genetically-modified]] virus.<ref name="mitvirusbattery">{{Cite web|url=http://web.mit.edu/newsoffice/2006/virus-battery.html|title=Researchers build tiny batteries with viruses|accessdate=2007-04-05|publisher=MIT News Office}}</ref> The MIT team was able to use the virus to create a working [[Battery (electricity)|battery]] with an [[energy density]] up to three times more than current materials. The potential exists for this technology to be used in [[liquid crystal]]s, [[solar cell]]s, [[fuel cells]], and other electronics in the future.
		+
		+	===Weapons===
		+	{{details|Biological warfare}}
		+	The ability of viruses to cause devastating [[epidemic]]s in human societies has led to the concern that viruses could be weaponized for [[biological warfare]]. Further concern was raised by the successful recreation of the infamous 1918 influenza virus in a laboratory.<ref name="cdcnews">{{Cite web|url=http://www.cdc.gov/OD/OC/MEDIA/pressrel/r051005.htm|title=Researchers Reconstruct 1918 Pandemic Influenza Virus; Effort Designed to Advance Preparedness|accessdate=2007-04-05|publisher=Centers for Disease Control}}</ref> The [[smallpox]] virus devastated numerous societies throughout history before its eradication. It currently exists in several secure laboratories in the world, and fears that it may be used as a weapon are not totally unfounded. The vaccine for smallpox is not safe and during the years before the eradication of smallpox disease more people became seriously ill as a result of vaccination than did people from smallpox.<ref name="pmid12911836">{{cite journal |author=Aragón TJ, Ulrich S, Fernyak S, Rutherford GW |title=Risks of serious complications and death from smallpox vaccination: a systematic review of the United States experience, 1963-1968 |journal=BMC public health |volume=3 |issue= |pages=26 |year=2003 |pmid=12911836 |doi=10.1186/1471-2458-3-26}}</ref> and smallpox vaccination is no longer practiced.<ref name="pmid15578369">{{cite journal |author=Weiss MM, Weiss PD, Mathisen G, Guze P |title=Rethinking smallpox |journal=Clin. Infect. Dis. |volume=39 |issue=11 |pages=1668–73 |year=2004 |pmid=15578369 |doi=10.1086/425745}}</ref> Thus, the modern global human population has almost no established resistance to smallpox; if it were to be released, a massive loss of life could be sustained before the virus is brought under control.
		+
		+	===See also===
		+	*[[Influenza]]
		+	*[[Rotavirus]]
		+	*[[Herpes simplex virus]]
		+	*[[Hepatitis B virus]]
		+	*[[Satellite (biology)|Satellite]]
		+
		+	==Electron micrographs of viruses==
		+	<gallery>
		+	Image:Norwalk.jpg|[[Norovirus]]. This RNA virus causes winter vomiting disease. It is often in the news as a cause of gastro-enteritis on cruise ships and in hospitals.
		+
		+	Image:Caliciviruses2.jpg|Caliciviruses are related to Noroviruses.
		+
		+	Image:Human Torovirus.jpg|Torovirus. An enveloped RNA virus.
		+
		+	Image:Coronaviruses 004 lores.jpg|Coronaviruses are a group of viruses that have a halo, or crown-like (corona) appearance when viewed under a microscope.
		+
		+	Image:Ebola virus em.png|Ebola Virus is a filamentous RNA virus.
		+
		+	Image:Measles virus.JPG|[[Measles]] virus. This is called a ''thin section'' where the virus particle has been cut in two.
		+
		+	Image:Respiratory syncytial virus 01.jpg|[[Respiratory Syncytial Virus]] (RSV). In this preparation the ribonucleoprotein can be seen as a herring bone pattern.
		+
		+	Image:Parvovirus in Blood.jpg|[[Parvovirus B19]]. Parvovirus B19 is a small DNA virus best known for causing a childhood exanthema called fifth disease or erythema infectiosum.
		+
		+	Image:Papilloma Virus (HPV) EM.jpg|[[Human Papilloma Virus]]
		+
		+	Image:EM of influenza virus.jpg|[[Influenza virus]]
		+
		+	Image:Herpes simplex virus TEM B82-0474 lores.jpg|Transmission electron micrograph of [[Herpes]] virus an enveloped virus that looks like fried eggs by negative stain electron microscopy.
		+
		+	Image:Polio EM PHIL 1875 lores.PNG|[[Transmission electron microscopy|TEM]] [[micrograph]] of [[Poliovirus]] virions.
		+
		+	</gallery>
		+
		+	==References==
		+	{{Reflist|2}}
		+
		+	[[Category:Virology|*]]
		+	[[Category:Viruses| ]]
		+
		+	{{Link FA|ja}}
	[[af:Virus]]		[[af:Virus]]
	[[als:Virus (Medizin)]]		[[als:Virus (Medizin)]]
	[[ar:فيروس]]		[[ar:فيروس]]
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	[[es:Virus]]		[[es:Virus]]
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	[[eu:Birus]]		[[eu:Birus]]
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-	[[fi:Virukset]]
	[[fo:Virus]]		[[fo:Virus]]
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		+	[[ko:바이러스]]
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	[[id:Virus]]		[[id:Virus]]
	[[is:Veira]]		[[is:Veira]]
	[[it:Virus (biologia)]]		[[it:Virus (biologia)]]
-	[[ja:ウイルス]]	+	[[he:נגיף]]
	[[ka:ვირუსები]]		[[ka:ვირუსები]]
-	[[ko:바이러스]]
	[[la:Virus biologicum]]		[[la:Virus biologicum]]
-	[[lt:Virusas]]
	[[lv:Vīruss]]		[[lv:Vīruss]]
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	[[mk:Вирус]]		[[mk:Вирус]]
-	[[mn:Вирус]]
	[[mr:विषाणू]]		[[mr:विषाणू]]
	[[ms:Virus]]		[[ms:Virus]]
		+	[[mn:Вирус]]
	[[nl:Virus (biologie)]]		[[nl:Virus (biologie)]]
-	[[nn:Virus]]	+	[[ja:ウイルス]]
	[[no:Virus]]		[[no:Virus]]
		+	[[nn:Virus]]
	[[oc:Virus]]		[[oc:Virus]]
	[[pl:Wirusy]]		[[pl:Wirusy]]
	[[pt:Vírus]]		[[pt:Vírus]]
-	[[qu:Añaw]]
	[[ro:Virus]]		[[ro:Virus]]
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	[[su:Virus]]		[[su:Virus]]
		+	[[fi:Virukset]]
	[[sv:Virus]]		[[sv:Virus]]
	[[ta:தீ நுண்மம்]]		[[ta:தீ நுண்மம்]]
	[[te:వైరస్]]		[[te:వైరస్]]
	[[th:ไวรัส]]		[[th:ไวรัส]]
		+	[[vi:Virus]]
	[[tr:Virüs]]		[[tr:Virüs]]
	[[uk:Вірус]]		[[uk:Вірус]]
	[[ur:حُمہ]]		[[ur:حُمہ]]
-	[[vi:Virus]]
	[[yi:ווירוס]]		[[yi:ווירוס]]
	[[zh:病毒]]		[[zh:病毒]]
-	[[zh-min-nan:Pēⁿ-to̍k]]

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Modèle:Otheruses align="right" rules="all" cellpadding="3" cellspacing="0" style="margin: 0 0 1em 1em; border: 1px solid #999; background-color:#F8F8F8;" ! colspan="2" style="background:#999999;" align="center" |<imagemap>Image:Information-silk.png|Comment lire une taxobox rect 0 0 50 50 Wikipédia:Lecture d'une taxobox desc none</imagemap>{{{4}}} |- | colspan="2" align="center" style="padding:0px;background:#CCCCCC;" | Image:Defaut.svg |- ! colspan="2" align="center" style="padding:1px;" | {{{6}}} |- ! colspan="2" style="background:#CCCCCC;" | [[{{{7}}}|Classification classique]] |-

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Modèle:Seealso

A virus (from the Latin virus meaning toxin or poison) is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell. Each viral particle, or virion, consists of genetic material, DNA or RNA, within a protective protein coat called a capsid. The capsid shape varies from simple helical and icosahedral (polyhedral or near-spherical) forms, to more complex structures with tails or an envelope. Viruses infect cellular life forms and are grouped into animal, plant and bacterial types, according to the type of host infected.

It has been argued whether viruses are living organisms. Some consider them non-living as they do not meet the criteria of the definition of life. For example, unlike most organisms, viruses do not have cells. However, viruses have genes and evolve by natural selection. They have been described as organisms at the edge of life. Viral infections in human as well as animal hosts, usually result in an immune response and disease. Often, a virus is completely eliminated by the immune system. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccines that produce lifelong immunity can prevent virus infections.

Sommaire 1 Etymology 2 History and discovery of viruses 3 Origins 4 Classification 4.1 ICTV classification 4.2 Baltimore Classification 5 Structure 5.1 Electron Microscopy 5.2 Size 6 Genome 6.1 Nucleic acid 6.2 Shape 6.3 Strandedness 6.4 Sense 6.5 Genome size 6.6 Gene reassortment 6.7 Genetic recombination 6.8 Genetic change 7 Replication 7.1 Virus life cycle 7.1.1 DNA viruses 7.1.2 RNA viruses 7.1.3 Reverse transcribing viruses 7.1.4 Bacteriophages 8 Lifeform debate 9 Viruses and disease 9.1 Epidemiology 9.2 Epidemics and pandemics 9.3 Viruses and cancer 9.4 Laboratory diagnosis 9.5 Prevention and treatment 9.6 Host immune response 9.7 Vaccines 9.8 Antiviral drugs 10 Applications 10.1 Life sciences and Medicine 10.2 Materials science and nanotechnology 10.3 Weapons 10.4 See also 11 Electron micrographs of viruses 12 References

Etymology

The word is from the Latin virus referring to poison and other noxious substances, first used in English in 1392.<ref name=Etymology_Dictionary> virus

. The Online Etymology Dictionary

 

. Retrieved on 2007-07-16. </ref> Virulent, from Latin virulentus "poisonous" dates to 1400.<ref name=OED> virulent, a.

. The Oxford English Dictionary - Online

 

. Retrieved on 2007-07-16. </ref> A meaning of "agent that causes infectious disease" is first recorded in 1728,<ref name=Etymology_Dictionary /> before the discovery of viruses by the Russian-Ukrainian biologist Dmitry Ivanovsky in 1892. The adjective viral dates to 1948.<ref name=OED2> viral, a.

. The Oxford English Dictionary - Online

 

. Retrieved on 2007-07-16. </ref> Today, virus is used to describe the biological viruses discussed above and as a metaphor for other parasitically-reproducing things, such as memes or computer viruses (since 1972).<ref name=OED /> The term virion is also used to refer to a single infective viral particle. The English plural form of virus is viruses.

History and discovery of viruses

Viral diseases such as rabies, yellow fever and smallpox have affected humans for centuries. There is hieroglyphical evidence of polio in the ancient Egyptian empire,<ref>Paul GF. (1971) A History of Poliomyelitis. Yale University Press: New Haven and London.</ref> though the cause of this disease was unknown at the time. In 1717, Mary Montagu, the wife of an English ambassador to the Ottoman Empire, observed local women inoculating their children against smallpox.<ref name=Behbehani_1983>Modèle:Cite journal</ref> In the late 18th century, Edward Jenner observed and studied Miss Sarah Nelmes, a milkmaid who had previously caught cowpox and was found to be immune to smallpox, a similar, but devastating virus. Jenner developed the first vaccine based on these findings. After lengthy vaccination campaigns, the World Health Organization (WHO) certified the eradication of smallpox in 1979.

Viruses themselves were discovered through a combination of new technology and old process of elimination. In the late 19th century Charles Chamberland developed a porcelain filter with pores small enough to remove cultured bacteria from their culture medium.<ref name=Horzinek_1997>Modèle:Cite journal</ref> Dimitri Ivanovski used this filter to study an infection of tobacco plants, now known as tobacco mosaic virus. He passed crushed leaf extracts of infected tobacco plants through the filter, then used the filtered extracts to infect other plants, thereby proving that the infectious agent was not a bacterium. Similar experiments were performed by several other researchers, with similar results. These experiments showed that viruses are orders of magnitudes smaller than bacteria. The term virus was coined by the Dutch microbiologist Martinus Beijerinck who showed, using methods based on the work of Ivanovski, that tobacco mosaic disease is caused by something smaller than a bacterium. He developed the term "contagium vivum fluidum" which means “soluble living germ” as first the idea of the virus.<ref>Chung, King-Thom and Ferris, Deam Hunter (1996). Martinus Willem Beijerinck (1851-1931): pioneer of general microbiology. AMS News 62, 539-543. http://www.asm.org/ASM/files/CCLIBRARYFILES/FILENAME/0000000251/621096p539.pdf PDF]</ref>

In the early 20th century, Frederick Twort discovered that bacteria could be infected by viruses.<ref> href="http://encyclopedia.jrank.org/Cambridge/entries/067/Frederick-William-Twort.html">Frederick William Twort</ref> Felix d'Herelle, working independently, showed that a preparation of viruses caused areas of cellular death on thin cell cultures spread on agar. Counting the dead areas allowed him to estimate the original number of viruses in the suspension. The invention of Electron microscopy provided the first look at viruses. In 1935 Wendell Stanley crystallised the tobacco mosaic virus and found it to be mostly protein.<ref name="pmid17756690">Modèle:Cite journal</ref> A short time later the virus was separated into protein and nucleic acid parts.<ref name="pmid17788438">Modèle:Cite journal</ref><ref name="pmid16590772">Modèle:Cite journal</ref> In 1939, Max Delbrück and E.L. Ellis demonstrated that, in contrast to cellular organisms, bacteriophage reproduce in "one step", rather than exponentially.<ref name="pmid16791793">Modèle:Cite journal</ref>

A major problem for early virologists was the inability to propagate viruses on sterile culture media, as is done with cellular microorganisms. This limitation required medical virologists to infect living animals with infectious material, which is dangerous. The first breakthrough came in 1931, when Ernest William Goodpasture demonstrated the growth of influenza and several other viruses in fertile chicken eggs.<ref name="pmid17810781">Modèle:Cite journal</ref> However, some viruses would not grow in chicken eggs, and a more flexible technique was needed for propagation of viruses. The solution came in 1949 when John Franklin Enders, Thomas H. Weller and Frederick Chapman Robbins together developed a technique to grow polio virus in cultures of living animal cells.<ref name="pmid15470207">Modèle:Cite journal</ref> Their methods have since been extended and applied to the growth of viruses and other infectious agents that do not grow on sterile culture media.

Origins

The origin of modern viruses is not entirely clear. It may be that no single mechanism can account for their origin.<ref>Holmes EC, Drummond AJ. The evolutionary genetics of viral emergence.Curr Top Microbiol Immunol. 2007;315:51-66.</ref> They do not fossilize well, so molecular techniques have been the most useful means of hypothesising how they arose.<ref>Liu Y, Nickle DC, Shriner D, Jensen MA, Learn GH Jr, Mittler JE, Mullins JI. Molecular clock-like evolution of human immunodeficiency virus type 1.Virology. 2004 Nov 10;329(1):101-8.</ref> Research in microfossil identification and molecular biology may yet discern fossil evidence dating to the Archean or Proterozoic eons. Two main hypotheses currently exist.<ref name="prescott">Modèle:Cite book</ref>

Small viruses with only a few genes may be runaway stretches of nucleic acid originating from the genome of a living organism. Their genetic material could have been derived from transferable genetic elements such as plasmids or transposons, that are prone to moving within, leaving, and entering genomes. New viruses are emerging de novo and therefore, it is not always the case that viruses have "ancestors"<ref>Keese P, Gibbs A. Plant viruses: master explorers of evolutionary space.Curr Opin Genet Dev. 1993 Dec;3(6):873-7.</ref>

Viruses with larger genomes, such as poxviruses, may have once been small cells that parasitised larger host cells. Over time, genes not required by their parasitic lifestyle would have been lost in a streamlining process known as retrograde-evolution or reverse-evolution. The bacteria Rickettsia and Chlamydia are living cells that, like viruses, can only reproduce inside host cells. They lend credence to the streamlining hypothesis, as their parasitic lifestyle is likely to have caused the loss of genes that enabled them to survive outside a host cell.

It is possible that viruses represent a primitive form of self replicating DNA and are a precursor to life as it is presently defined.<ref>Koonin EV. The Biological Big Bang model for the major transitions in evolution.Biol Direct. 2007 Aug 20;2:21.</ref> Other infectious particles which are even simpler in structure than viruses include viroids, satellites, and prions.

Classification

Modèle:Details In taxonomy, the classification of viruses is difficult owing to the lack of a fossil record and the dispute over whether they are living or non-living.<ref> Rybicki EP (1990) The classification of organisms at the edge of life, or problems with virus systematics. S Aft J Sci 86:182-186</ref><ref name="pmid13481308">Modèle:Cite journal</ref> They do not fit easily into any of the domains of biological classification and classification begins at the family rank. However, the domain name of Acytota (without cells) has been suggested. This would place viruses on a par with the other domains of Eubacteria, Archaea, and Eukarya. Not all families are currently classified into orders, nor all genera classified into families.

In 1962 André Lwoff, Robert Horne, and Paul Tournier were the first to develop a means of virus classification, based on the Linnaean hierarchical system.<ref name="pmid14467544">Modèle:Cite journal</ref> This system based classification on phylum, class, order, family, genus, and species. Viruses were grouped according to their shared properties (not of their hosts) and the type of nucleic acid forming their genomes.<ref name="pmid13931895">Modèle:Cite journal</ref> Following this initial system, a few modifications were made and the International Committee on Taxonomy of Viruses was developed (ICTV).

ICTV classification

The International Committee on Taxonomy of Viruses (ICTV) developed the current classification system and put in place guidelines that put a greater weighting on certain virus properties to maintain family uniformity. A universal system for classifying viruses, and a unified taxonomy, has been established since 1966. In determining order, taxonomists should consider the type of nucleic acid present, whether the nucleic acid is single- or double-stranded, and the presence or absence of an envelope. After these three main properties, other characteristics can be considered: the type of host, the capsid shape, immunological properties and the type of disease it causes. The system makes use of a series of ranked taxons. The general structure is as follows:

Order (-virales)

Family (-viridae)

Subfamily (-virinae)

Genus (-virus)

Species (-virus)

The recognition of orders is very recent; to date, only 3 have been named, most families remain unplaced. The committee does not formally distinguish between subspecies, strains, and isolates. In total there are 3 orders, 56 families, 9 subfamilies, 233 genera. ICTV recognizes about 1,550 virus species but about 30,000 virus strains and isolates are being tracked by virologists.<ref> Virus Taxonomy 8th Reports of the International Committee on Taxonomy of Viruses C.M. Fauquet, M.A. Mayo, J. Maniloff, U. Desselberger, and L.A. Ball (eds) Academic Press, 1162 pp. (2005) Elsevier Publication Date: 27 May 2005 </ref>

The Nobel Prize-winning biologist David Baltimore devised the Baltimore classification system.<ref name="pmid4377923">Modèle:Cite journal</ref><ref name="pmid4348509">Modèle:Cite journal</ref> The ICTV classification system is used in conjunction with the Baltimore classification system in modern virus classification.<ref name="pmid15078590">Modèle:Cite journal</ref><ref name="pmid10486120">Modèle:Cite journal</ref><ref name="pmid15183049">Modèle:Cite journal</ref>

Baltimore Classification

The Baltimore classification of viruses is based on the mechanism of mRNA production. Viruses must generate positive strand mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. This classification places viruses into seven groups:

• I: Double-stranded DNA (e.g. Adenoviruses, Herpesviruses, Poxviruses)

• II: Single-stranded (+)sense DNA (e.g. Parvoviruses)

• III: Double-stranded RNA (e.g. Reoviruses)

• IV: Single-stranded (+)sense RNA (e.g. Picornaviruses, Togaviruses)

• V: Single-stranded (-)sense RNA (e.g. Orthomyxoviruses, Rhabdoviruses)

• VI: Single-stranded (+)sense RNA with DNA intermediate in life-cycle (e.g. Retroviruses)

• VII: Double-stranded DNA with RNA intermediate (e.g. Hepadnaviruses)

As an example of viral classification, the chicken pox virus, Varicella zoster (VZV), belongs to family Herpesviridae, subfamily Alphaherpesvirinae and genus Varicellovirus. It remains unranked in terms of order. VZV is in Group I of the Baltimore Classification because it is a dsDNA virus that does not use reverse transcriptase.

Structure

A complete virus particle, known as a virion, consists of nucleic acid surrounded by a protective coat of protein called a capsid. Viruses can have a lipid 'envelope' derived from the host cell membrane. A capsid is made from proteins encoded by the viral genome and its shape serves as the basis for morphological and antigenic distinction.<ref name="pmid14019094">Modèle:Cite journal</ref><ref name="pmid13309339">Modèle:Cite journal</ref> Virally coded protein subunits will self-assemble to form a capsid, generally requiring the presence of the virus genome. However, complex viruses code for proteins which assist in the construction of their capsid.<ref name=prescott> </ref> Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid.

In general, there are four main morphological virus types:

Helical viruses
Image:Tobacco mosaic virus structure.png Diagram of a helical capsid	Helical capsids are composed of a single type of subunit stacked around a central axis to form a helical structure which may have a central cavity, or hollow tube. This arrangement results in rod-shaped or filamentous virions: these can be anything from short and highly rigid, to long and very flexible. The genetic material, generally single-stranded RNA, but ssDNA in some cases, is bound into the protein helix, by interactions between the negatively-charged nucleic acid and positive charges on the protein. Overall, the length of a helical capsid is related to the length of the nucleic acid contained within it and the diameter is dependent on the size and arrangement of protomers. The well-studied Tobacco mosaic virus is an example of a helical virus.
Icosahedral viruses
Image:Enteric Adenoviruses.jpg Electron micrograph of icosahedral virions	Icosahedral capsid symmetry results in a spherical appearance of viruses at low magnification but actually consists of capsomers arranged in a regular geometrical pattern, similar to a soccer ball, hence they are not truly "spherical". Capsomers are ring shaped constructed from five to six copies of protomers. These associate via non-covalent bonding to enclose the viral nucleic acid, though generally less intimately than helical capsids, and may involve one or more protomers. Icosahedral architecture was employed by R. Buckminster-Fuller in his geodesic dome, and is the most efficient way of creating an enclosed robust structure from multiple copies of a single protein. The number of proteins required to form a spherical virus capsid is denoted by the T-number,<ref name="triang"> Virus triangulation numbers via Internet Archive . Retrieved on 2006-04-05. </ref> where 60×t proteins are necessary. In the case of the hepatitis B virus the T-number is 4, and 240 proteins assemble to form the capsid.
Enveloped viruses
Image:Chickenpox-virus.jpg Herpes zoster virus	Viruses are able to envelope themselves in a modified form of one of the cell membranes either the outer membrane surrounding an infected host cell, or from internal membranes such as nuclear membrane or endoplasmic reticulum, thus gaining an outer lipid bilayer known as a viral envelope. This membrane is studded with proteins coded for by the viral genome and host genome; however the lipid membrane itself and any carbohydrates present are entirely host-coded. The Influenza virus and HIV use this strategy. The viral envelope can give a virion a few distinct advantages over other capsid-only virions, such as protection from enzymes and certain chemicals. The proteins in it can include glycoproteins functioning as receptor molecules, allowing host cells to recognise and bind these virions, resulting in the possible uptake of the virion into the cell. Most enveloped viruses are dependent on the envelope for infectivity.
Complex viruses
Image:Tevenphage.png Diagram of a bacteriophage	These viruses possess a capsid which is neither purely helical, nor purely icosahedral, and which may possess extra structures such as protein tails or a complex outer wall. Some bacteriophages have a complex structure consisting of an icosahedral head bound to a helical tail, the latter of which may have a hexagonal base plate with protruding protein tail fibres.
Image:Poxvirus.jpg Poxvirus	The Poxviruses are large, complex viruses which have an unusual morphology. The viral genome is associated with proteins within a central disk structure known as a nucleoid. The nucleoid is surrounded by a membrane and two lateral bodies of unknown function. The virus has an outer envelope with a thick layer of protein studded over its surface. The whole particle is slightly pleiomorphic, ranging from ovoid to brick shape.<ref>Long GW, Nobel J, Murphy FA, Herrmann KL, and Lourie B (1970) Experience with electron microscopy in the differential diagnosis of smallpox. Applied Microbiology 20(3):497-504.</ref>

Electron Microscopy

Modèle:Details

Electron Microscopy is the commonest method used to study the morphology of viruses. To increase the contrast between viruses and the background, electron-dense "stains" are used. These are solutions of salts of heavy metals such as tungsten, that scatter the electrons from regions covered with the stain. When virus particles are coated with stain (positive staining), fine detail is obscured. Negative staining overcomes this problem by staining the background only.<ref name="pmid1715774">Modèle:Cite journal</ref>

Size

A medium sized virion next to a flea is roughly equivalent to a human next to a mountain twice the size of Mount Everest. Some filoviruses have a total length of up to 1400 nm, however their capsid diameters are only about 80 nm. Most viruses which have been studied have a capsid diameter between 10 and 300 nanometres. Most viruses are unable to be seen with a light microscope but some are as large or larger than the smallest bacteria and can be seen under high optical magnification. More commonly, both scanning and transmission electron microscopes are used to visualise virus particles.

Genome

Genomic diversity among viruses

Property	Parameters
Nucleic acid	DNA RNA Both DNA and RNA
Shape	Linear Circular Segmented
Strandedness	Single-stranded Double-stranded Double-stranded with regions of single-strandedness
Sense	Positive sense (+) Negative sense (-) Ambisense (+/-)

An enormous variety of genomic structures can be seen among viral species; as a group they contain more structural genomic diversity than the entire kingdoms of either plants, animals, or bacteria.<ref name="flint">Modèle:Cite book</ref>

Nucleic acid

A virus may employ either DNA or RNA as the nucleic acid. Rarely do they contain both, however cytomegalovirus is an exception to this, possessing a DNA core with several mRNA segments.<ref name=prescott> </ref> By far most viruses have RNA. Plant viruses tend to have single-stranded RNA and bacteriophages tend to have double-stranded DNA.<ref name="prescott"> </ref> Some virus species possess abnormal nucleotides, such as hydroxymethylcytosine instead of cytosine, as a normal part of their genome.<ref name="prescott"> </ref>

Shape

Viral genomes may be circular, such as polyomaviruses, or linear, such as adenoviruses. The type of nucleic acid is irrelevant to the shape of the genome. Among RNA viruses, the genome is often divided up into separate parts within the virion and are called segmented. Double-stranded RNA genomes and some single-stranded RNA genomes are segmented.<ref name=prescott> </ref> Each segment often codes for one protein and they are usually found together in one capsid. Every segment is not required to be in the same virion for the overall virus to be infectious, as demonstrated by the brome mosaic virus.<ref name="prescott"> </ref>

Strandedness

A viral genome, irrespective of nucleic acid type, may be either single-stranded or double-stranded. Single-stranded genomes consist of an unpaired nucleic acid, analogous to one-half of a ladder split down the middle. Double-stranded genomes consist of 2 complementary paired nucleic acids, analogous to a ladder. Viruses, such as those belonging to the Hepadnaviridae, contain a genome which is partially double-stranded and partially single-stranded.<ref name="flint"> </ref> Viruses that infect humans include double-stranded RNA (e.g. Rotavirus), single-stranded RNA (e.g. Influenza virus), single-stranded DNA (e.g. Parvovirus B19) and double-stranded DNA (Herpes virus).

Sense

For viruses with RNA as their nucleic acid, the strands are said to be either positive-sense (called the plus-strand) or negative-sense (called the minus-strand) depending on whether it is complementary to viral mRNA. Positive-sense viral RNA is identical to viral mRNA and thus can be immediately translated by the host cell. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA polymerase before translation. DNA nomenclature is similar to RNA nomenclature, in that the coding strand for the viral mRNA is complementary to it (-), and the non-coding strand is a copy of it (+).

Genome size

Genome size in terms of the weight of nucleotides varies between species. The smallest genomes code for only four proteins and weigh about 10⁶ Daltons, the largest weigh about 10⁸ Daltons and code for over one hundred proteins.<ref name=prescott> </ref> RNA viruses generally have smaller genome sizes than DNA viruses due to a higher error-rate when replicating, resulting in a maximum upper size limit. Beyond this limit, errors in the genome when replicating render the virus useless or uncompetitive. To compensate for this, RNA viruses often have segmented genomes where the genome is split into smaller molecules, thus reducing the chance of error.<ref>Pressing J, Reanney DC. Divided genomes and intrinsic noise.J Mol Evol. 1984;20(2):135-46.</ref> In contrast, DNA viruses generally have larger genomes due to the high fidelity of their replication enzymes.<ref name="flint"> </ref>

Gene reassortment

There is an evolutionary advantage in having a segmented genome. Different strains of a virus with a segmented genome, from a pig or a bird or a human for example, such as Influenza virus, can shuffle and combine with other genes producing progeny viruses or (offspring) that have unique characteristics. This is called reassortment or viral sex.<ref> Goudsmit, Jaap. Viral Sex. Oxford Univ Press, 1998.ISBN-13: 9780195124965 ISBN-10: 0195124960</ref> This is one reason why Influenza virus constantly changes.<ref> Zhou NN, Senne DA, Landgraf JS, Swenson SL, Erickson G, Rossow K, Liu L, Yoon K, Krauss S, Webster RG. Genetic reassortment of avian, swine, and human influenza A viruses in American pigs.J Virol. 1999 Oct;73(10):8851-6.</ref>

Genetic recombination

Genetic recombination is the process by which a strand of DNA is broken and then joined to the end of a different DNA molecule. This can occur when viruses infect cells simultaneously and studies of viral evolution have shown that recombination has been rampant in the species studied.<ref name="pmid10573145">Modèle:Cite journal</ref> Recombination is common to both RNA and DNA viruses.<ref name="pmid15578739">Modèle:Cite journal</ref><ref name="pmid10479778">Modèle:Cite journal</ref>

Genetic change

Viruses undergo genetic change by several mechanisms. These include a process called genetic drift where individual bases in the DNA or RNA mutate to other bases. Most of these point mutations are silent in that they do not change the protein that the gene encodes, but others can confer evolutionary advantages such as resistance to antiviral drugs.<ref>Pan XP, Li LJ, Du WB, Li MW, Cao HC, Sheng JF. Differences of YMDD mutational patterns, precore/core promoter mutations, serum HBV DNA levels in lamivudine-resistant hepatitis B genotypes B and C. J Viral Hepat. 2007 Nov;14(11):767-74.</ref> Antigenic shift where there is a major change in the genome of the virus. This occurs as a result of recombination or reassortment (see above). When this happens with influenza viruses, pandemics may result.<ref>Hampson AW, Mackenzie JS. The influenza viruses.Med J Aust. 2006 Nov 20;185(10 Suppl):S39-43.</ref><ref>Nakajima K. The mechanism of antigenic shift and drift of human influenza virus Nippon Rinsho. 2003 Nov;61(11):1897-903.</ref> By genome rearrangement where the structure of the gene changes although no mutations have necessarily occurred.<ref>Hundley F, McIntyre M, Clark B, Beards G, Wood D, Chrystie I, Desselberger U. Heterogeneity of genome rearrangements in rotaviruses isolated from a chronically infected immunodeficient child.J Virol. 1987 Nov;61(11):3365-72.</ref>

RNA viruses are much more likely to mutate than DNA viruses for the reasons outlined above. Viruses often exist as quasispecies or swarms of viruses of the same species but with slightly different genome nucleoside sequences. Such quasispecies are a prime target for natural selection.<ref>Metzner KJ. Detection and significance of minority quasispecies of drug-resistant HIV-1. HIV Ther. 2006 Dec;11(4):74-81.</ref>

Replication

Viral populations do not grow through cell division, because they are acellular; instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves. A virus can still cause degenerative effects within a cell without causing its death; collectively these are termed cytopathic effects.

Virus life cycle

The life cycle of viruses differs greatly between species (see below) but there are six basic stages in the life cycle of viruses:

• Attachment is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. This specificity determines the host range of a virus. For example, the human immunodeficiency virus (HIV) infects only human T cells, because its surface protein, gp120, can interact with CD4 and receptors on the T cell's surface. This mechanism has evolved to favour those viruses that only infect cells that they are capable of replicating in. Attachment to the receptor can induce the viral-envelope protein to undergo changes that results in the fusion of viral and cellular membranes.
• Penetration: following attachment, viruses enter the host cell through receptor mediated endocytosis or membrane fusion.
• Uncoating is a process that viral capsid is removed is degraded by viral enzymes or host enzymes thus releasing the viral genomic nucleic acid.
• Replication involves synthesis of viral messenger RNA (mRNA) for viruses except positive sense RNA viruses (see above), viral protein synthesis and assembly of viral proteins and viral genome replication.
• Following the assembly of the virus particles post-translational modification of the viral proteins often occurs. In viruses such as HIV, this modification, (sometimes called maturation), occurs after the virus has been released from the host cell.<ref name="pmid11451488">Modèle:Cite journal</ref>
• Viruses are released from the host cell by lysis (see below) . Enveloped viruses (e.g., HIV) typically are released from the host cell by “budding”. During this process, the virus acquires its phospholipid envelope which contains embedded viral glycoproteins.

DNA viruses

Animal DNA viruses, such as herpesviruses, enter the host via endocytosis, the process by which cells take in material from the external environment. Frequently after a chance collision with an appropriate surface receptor on a cell, the virus penetrates the cell, the viral genome is released from the capsid and host polymerases begin transcribing viral mRNA. New virions are assembled and released either by cell lysis or by budding off the cell membrane.

RNA viruses

Animal RNA viruses can be placed into about four different groups depending on their modes of replication. The polarity of the RNA largely determines the replicative mechanism, as well as whether the genetic material is single-stranded or double-stranded. Some RNA viruses are actually DNA based but use an RNA-intermediate to replicate. RNA viruses are dependent on virally encoded RNA replicase to create copies of their genomes.

Reverse transcribing viruses

Reverse transcribing viruses replicate using reverse transcription, which is the formation of DNA from an RNA template. Viruses containing RNA genomes use a DNA intermediate to replicate, whereas those containing DNA genomes use an RNA intermediate during genome replication. Both types use the reverse transcriptase enzyme to carry out the nucleic acid conversion. both types are susceptible to antiviral drugs that inhibit the reverse transcriptase enzyme, e.g. zidovudine and lamivudine.

An example of the first type is HIV which is a retrovirus. Retroviruses often integrate the DNA produced by reverse transcription into the host genome. This is why HIV infection can at present, only be treated and not cured.

Examples of the second type are the Hepadnaviridae which includes Hepatitis B virus and the Caulimoviridae - e.g. Cauliflower mosaic virus.

Bacteriophages

Main article: Bacteriophage

Bacteriophages infect specific bacteria by binding to surface receptor molecules and then enter the cell. Within a short amount of time, in some cases, just minutes, bacterial polymerase starts translating viral mRNA into protein. These proteins go on to become either new virions within the cell, helper proteins which help assembly of new virions, or proteins involved in cell lysis. Viral enzymes aid in the breakdown of the cell membrane, and in the case of the T4 phage, in just over twenty minutes after injection over three hundred phages could be released.

Lifeform debate

Viruses have been described as organisms at the edge of life<ref>Rybicki ibid</ref> but argument continues over whether viruses are truly alive. According to the United States Code, they are considered microorganisms in the sense of biological weaponry and malicious use. Scientists, however, are divided. Things become more complicated as they look at viroids and prions. Viruses resemble other organisms in that they possess genes, and can evolve–in infected cells–by natural selection.<ref>Holmes EC.PLoS Biol. 2007 Oct 2;5(10):e278. Viral Evolution in the Genomic Age</ref><ref>Shackelton LA, Holmes EC.Phylogenetic evidence for the rapid evolution of human B19 erythrovirus.J Virol. 2006 Apr;80(7):3666-9.</ref> They can reproduce by creating multiple copies of themselves through self-assembly.

Viruses do not have a cell structure (regarded as the basic unit of life), although they do have genes. Additionally, although they reproduce, they do not self-metabolize and require a host cell to replicate and synthesise new products. However, bacterial species such as Rickettsia and Chlamydia, are considered living organisms, but are unable to reproduce outside a host cell.

An argument can be made that accepted forms of life use cell division to reproduce, whereas viruses spontaneously assemble within cells. The comparison is drawn between viral self-assembly and the autonomous growth of non-living crystals. Virus self-assembly within host cells has implications for the study of the origin of life, as it lends credence to the hypothesis that life could have started as self-assembling organic molecules.<ref>Vlassov, Alexander V. (Jul 2005). "The RNA World on Ice: A New Scenario for the Emergence of RNA Information". Journal of Molecular Evolution 61: 264-273. </ref>

If viruses are considered alive, then the criteria specifying life will have to exclude the cell. If viruses are said to be alive, the question could follow of whether even smaller infectious particles, such as viroids and prions, are alive.

Viruses and disease

For more examples of diseases caused by viruses see List of infectious diseases

Examples of common human diseases caused by viruses include the common cold, the flu, chickenpox and cold sores. Serious diseases such as Ebola, AIDS, avian influenza and SARS are caused by viruses. The relative ability of viruses to cause disease is described in terms of virulence. Other diseases are under investigation as to whether they too have a virus as the causative agent, such as the possible connection between Human Herpesvirus Six (HHV6) and neurological diseases such as multiple sclerosis and chronic fatigue syndrome. There is current controversy over whether the borna virus, previously thought of as causing neurological disease in horses, could be responsible for psychiatric illness in humans.<ref name=Chen_1999>Modèle:Cite journal</ref>

Viruses have different mechanisms by which they produce disease in an organism, which largely depends on the species. Mechanisms at the cellular level primarily include cell lysis, the breaking open and subsequent death of the cell. In multicellular organisms, if enough cells die the whole organism will start to suffer the effects. Although viruses cause disruption of healthy homeostasis, resulting in disease, they may exist relatively harmlessly within an organism. An example would include the ability of the herpes simplex virus, which cause coldsores, to remain in a dormant state within the human body. This is called latency<ref>Margolis TP, Elfman FL, Leib D, Pakpour N, Apakupakul K, Imai Y, Voytek C. Spontaneous reactivation of herpes simplex virus type 1 in latently infected murine sensory Ganglia.J Virol. 2007 Oct;81(20):11069-74. Epub 2007 Aug 8.</ref> and is a characteristic of the herpes viruses including Epstein-Barr virus which causes glandular fever and Varicella zoster virus which causes chicken pox. Latent chickenpox infections return in later life as the disease called shingles.

Some viruses can cause life-long or chronic infections where the viruses continue to replicate in the body despite the hosts' defense mechanisms.<ref name="pmid17931183">Modèle:Cite journal</ref> This is common in Hepatitis B virus and Hepatitis C Virus infections. People chronically infected with Hepatitis B virus are known as carriers who serve as reservoirs of infectious virus. In some populations, with a high proportion of carriers, the disease is said to be endemic.<ref name="pmid17645465">Modèle:Cite journal</ref> When diagnosing Hepatitis B virus infections it is important to distinguish between acute and chronic infections.<ref name="pmid17664817">Modèle:Cite journal</ref>

Epidemiology

Viral epidemiology is the branch of medical science dealing with the transmission and control of virus infections in humans. Transmission of viruses can be “vertical”, that is from mother to child, or “horizontal” which means from person to person. Examples of vertical transmission include Hepatitis B virus and HIV where the baby is born already infected with the virus.<ref name="pmid17825648">Modèle:Cite journal</ref> Another, more rare, example is Varicella zoster virus which although causing relatively mild infections in humans can be fatal to the foetus and newly born baby.<ref name="pmid11190597">Modèle:Cite journal</ref> Horizontal transmission is the commonest mechanism of spread of viruses in populations. Transmission can be exchange of blood, by sexual activity e.g. HIV and Hepatitis B and Hepatitis C, by mouth by exchange of saliva, e.g. Epstein-Barr virus or from contaminated food or water e.g. Norovirus, by breathing in viruses in the form of aerosols e.g. Influenza virus and by insect vectors such as mosquitoes e.g. dengue. The rate or speed of transmission of virus infections depends on factors that include population density, the number of susceptible individuals, (i.e. those who are not immune),<ref> Garnett GP. Role of herd immunity in determining the effect of vaccines against sexually transmitted disease.J Infect Dis. 2005 Feb 1;191 Suppl 1:S97-106.</ref> the quality of health care and the weather.<ref name="pmid16544901">Modèle:Cite journal</ref>

Epidemics and pandemics

Modèle:Details

Native American populations were devastated by contagious diseases, particularly smallpox, brought to the Americas by European colonists. It is unclear how many Native Americans were killed by foreign diseases after the arrival of Columbus in the Americas, but the numbers have been estimated to be close to 70% of the indigenous population. The damage done by this disease may have significantly aided European attempts to displace or conquer the native population.<ref>Ranlet P. The British, the Indians, and smallpox: what actually happened at Fort Pitt in 1763? Pa Hist. 2000;67(3):427-41.</ref><ref>Van Rijn K. "Lo! The poor Indian!" colonial responses to the 1862-63 smallpox epidemic inBritish Columbia and Vancouver Island.Can Bull Med Hist. 2006;23(2):541-60.</ref><ref>Patterson KB, Runge T. Smallpox and the Native American.Am J Med Sci. 2002 Apr;323(4):216-22.</ref><ref>Sessa R, Palagiano C, Scifoni MG, di Pietro M, Del Piano M.

The major epidemic infections: a gift from the Old World to the New?

Panminerva Med. 1999 Mar;41(1):78-84.</ref><ref>Bianchine PJ, Russo TA.

The role of epidemic infectious diseases in the discovery of America.

Allergy Proc. 1992 Sep-Oct;13(5):225-32.</ref><ref>Hauptman LM.

Smallpox and American Indian; Depopulation in Colonial New York.

N Y State J Med. 1979 Nov;79(12):1945-9.</ref><ref>Fortuine R.

Smallpox decimates the Tlingit (1787).

Alaska Med. 1988 May-Jun;30(3):109.</ref>

Main article: Spanish flu

A pandemic is a world-wide epidemic. The 1918 flu pandemic, commonly referred to as the Spanish flu, was a category 5 influenza pandemic caused by an unusually severe and deadly Influenza A virus. The victims were often healthy young adults, in contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or otherwise weakened patients.
The Spanish flu pandemic lasted from 1918 to 1919. Older estimates say it killed 40–50 million people<ref name=Patterson1>Modèle:Cite journal</ref>

Main article: Aids

Acquired Immune Deficiency Syndrome
Most researchers believe that HIV originated in sub-Saharan Africa during the twentieth century;<ref name=Gao>

Modèle:Cite journal</ref> it is now a pandemic, with an estimated 38.6 million people now living with the disease worldwide.<ref name=UNAIDS2006>Modèle:Cite book</ref> As of January 2006, the Joint United Nations Programme on HIV/AIDS (UNAIDS) and the World Health Organization (WHO) estimate that AIDS has killed more than 25 million people since it was first recognized on June 5, 1981, making it one of the most destructive epidemics in recorded history.<ref> Mawar N, Saha S, Pandit A, Mahajan U.

The third phase of HIV pandemic: social consequences of HIV/AIDS stigma &

discrimination & future needs.Indian J Med Res. 2005 Dec;122(6):471-84. Review.</ref>

Main article: Ebola

Several highly lethal viral pathogens are members of the Filoviridae. Filoviruses are filament-like viruses that cause viral hemorrhagic fever, and include the Ebola and Marburg viruses. The Marburg virus attracted widespread press attention in April 2005 for an outbreak in Angola. Beginning in October 2004 and continuing into 2005, the outbreak was the world's worst epidemic of any kind of viral hemorrhagic fever.<ref>Towner JS, Khristova ML, Sealy TK, Vincent MJ, Erickson BR, Bawiec DA, HartmanAL, Comer JA, Zaki SR, Stroher U, Gomes da Silva F, del Castillo F, Rollin PE,Ksiazek TG, Nichol ST.

Marburgvirus genomics and association with a large hemorrhagic fever outbreak in 

Angola.J Virol. 2006 Jul;80(13):6497-516.</ref>

Viruses and cancer

Modèle:Details

Viruses are an established cause of malignancy in humans and other species. The main viruses associated with human cancers are human papillomavirus, hepatitis B and hepatitis C virus, Epstein-Barr virus, and human T-lymphotropic virus. Hepatitis viruses, including hepatitis B and hepatitis C, can induce a chronic viral infection that leads to liver cancer.<ref> Modèle:Cite journal</ref><ref> Modèle:Cite journal </ref> Infection by Human T-lymphotropic virus can lead to Tropical Spastic Paraparesis and Adult T-cell leukaemia<ref> Modèle:Cite journal</ref> Human papillomaviruses are an established cause of cancers of cervix, skin, anus, and penis.<ref> Modèle:Cite journal</ref> Within the Herpesviridae, Kaposi’s sarcoma-associated herpesvirus causes Kaposi’s sarcoma and Body cavity lymphoma and Epstein–Barr virus causes Burkitt’s lymphoma, Hodgkin’s lymphoma, B lymphoproliferative disease and Nasopharyngeal carcinoma.<ref> Modèle:Cite journal</ref>

Laboratory diagnosis

In the diagnostic laboratory virus infections are confirmed by several methods that include:

• Growth of the virus in a cell culture from a specimen taken from the patient.

• Detection of virus-specific IgM antibody (see below) in the blood.

• Detection of virus antigens by ELISA in tissues and fluids.

• Detection of virus encoded DNA and RNA by PCR.

• Observation of virus particles by electron microscopy.

Prevention and treatment

Because viruses use the machinery of a host cell to reproduce and reside within them, they are difficult to eliminate without killing the host cell. The most effective medical approaches to viral diseases so far are vaccinations to provide resistance to infection, and antiviral drugs which treat the symptoms of viral infections.

Host immune response

The body's first line of defense against viruses is the innate immune system. This comprises cells and other mechanisms that defend the host from infection in a non-specific manner. This means that the cells of the innate system recognize, and respond to, pathogens in a generic way, but unlike the adaptive immune system, it does not confer long-lasting or protective immunity to the host.<ref name=Alberts>Modèle:Cite book</ref>

RNA interference is an important innate defense against viruses.<ref>Ding SW, Voinnet O. Antiviral immunity directed by small RNAs. Cell. 2007 Aug 10;130(3):413-26.</ref> Many viruses have a replication strategy that involves double-stranded RNA [dsRNA]. When such a virus infects a cell, it releases its RNA molecule or molecules, which immediately bind to a protein complex called Dicer that cuts the RNA into smaller pieces. A biochemical pathway called the RISC complex is activated which degrades the viral mRNA and the cell survives the infection. Rotaviruses avoid this mechanism by not uncoating fully inside the cell and by releasing newly produced mRNA through pores in the particles inner capsid. The genomic dsRNA remains protected inside the core of the virion.<ref name="pmid15579070">Modèle:Cite journal</ref><ref name="pmid15010218">Modèle:Cite journal</ref>

When the adaptive immune system of a vertebrate encounters a virus, it produces specific antibodies which bind to the virus and render it non-infectious. This is called humoral immunity. Two types of antibodies are important. The first called IgM is highly effective at neutralizing viruses but is only produced by the cells of the immune system for a few weeks. The second, called, IgG is produced indefinitely. The presence of IgM in the blood of the host is used to test for acute infection, whereas IgG indicates an infection sometime in the past.<ref>Greer S, Alexander GJ. Viral serology and detection. Baillieres Clin Gastroenterol. 1995 Dec;9(4):689-721</ref> Both types of antibodies are measured when tests for immunity are carried out.<ref>Laurence JC. Hepatitis A and B immunizations of individuals infected with humanimmunodeficiency virus.Am J Med. 2005 Oct;118 Suppl 10A:75S-83S.</ref>

A second defense of vertebrates against viruses is called cell-mediated immunity and involves immune cells known as T cells. The body's cells constantly display short fragments of their proteins on the cell's surface, and if a T cell recognizes a suspicious viral fragment there, the host cell is destroyed by T killer cells and the virus-specific T-cells proliferate. Cells such as the macrophage are specialists at this antigen presentation.<ref>Cascalho M, Platt JL. Novel functions of B cells.Crit Rev Immunol. 2007;27(2):141-51.</ref><ref>Khatri M, Sharma JM. Modulation of macrophages by infectious bursal disease virus.Cytogenet Genome Res. 2007;117(1-4):388-93</ref>

Not all virus infections produce a protective immune response in this way. HIV evades the immune system by constantly changing the amino acid sequence of the proteins on the surface of the virion. These persistent viruses evade immune control by sequestration, blockade of antigen presentation, cytokine resistance, evasion of natural killer cell activities, escape from apoptosis, and antigenic shift.<ref>Hilleman MR. Strategies and mechanisms for host and pathogen survival in acute and persistent viral infections.Proc Natl Acad Sci U S A. 2004 Oct 5;101 Suppl 2:14560-6. Epub 2004 Aug 5.</ref>

The production of interferon is an important host defense mechanism.<ref>Le Page C, Genin P, Baines MG, Hiscott J. Interferon activation and innate immunity.Rev Immunogenet. 2000;2(3):374-86.</ref>

Vaccines

Modèle:Details Vaccination is a cheap and effective way of preventing infections by viruses. Vaccines were used to prevent viral infections long before the discovery of the actual viruses. Their use has resulted in a dramatic decline in morbidity (illness) and mortality (death) associated with viral infections such as polio, measles, mumps and rubella.<ref name="pmid17068034">Modèle:Cite journal</ref> Smallpox infections have been eradicated.<ref name="pmid16989262">Modèle:Cite journal</ref> Currently vaccines are available to prevent over thirteen viral infections of humans<ref name="pmid16364754">Modèle:Cite journal</ref> and more are used to prevent viral infections of animals.<ref name="pmid17892169">Modèle:Cite journal</ref> Vaccines can consist of live or killed viruses.<ref name="pmid16494719">Modèle:Cite journal</ref> Live vaccines contain weakened forms of the virus that causes the disease. Such viruses are called attenuated. Live vaccines can be dangerous when given to people with a weak immunity, (who are described as immunocompromised), because in these people the weakened virus can cause the original disease.<ref name="pmid1090805">Modèle:Cite journal</ref> Biotechnology and genetic engineering techniques are used to produce subunit vaccines. These vaccines use only the capsid proteins of the virus. Hepatitis B vaccine is an example of this type of vaccine.<ref name="pmid3018891">Modèle:Cite journal</ref> Subunit vaccines are safe for immunocompromised patients because they cannot cause the disease.<ref name="pmid16221073">Modèle:Cite journal</ref>

Antiviral drugs

Modèle:Details

Over the past twenty years the development of antiviral drugs has increased rapidly. This has been driven by the AIDS epidemic. Antiviral drugs are often nucleoside analogues, (fake DNA building blocks), which viruses incorporate into their genomes during replication. The life-cycle of the virus is then halted because the newly synthesised DNA is inactive. This is because these analogues lack the hydroxyl groups which along with phosphorus atoms, link together to form the strong "backbone" of the DNA molecule. This is called DNA chain termination.<ref name="pmid15592828">Modèle:Cite journal</ref> Examples of nucleoside analogues are aciclovir for Herpes virus infections and lamivudine for HIV and Hepatitis B virus infections. Aciclovir, is one of the oldest and most frequently prescribed antiviral drugs.<ref name="pmid6355051">Modèle:Cite journal</ref>

Other antiviral drugs in use target different stages of the viral life cycle. HIV is dependent on a proteolytic enzyme called the HIV-1 protease for it to become fully infectious. There is a class of drugs called protease inhibitors which have been designed to inactivate the enzyme.

Hepatitis C is caused by an RNA virus. In 80% of people infected the disease is chronic and without treatment they are infected and infectious for the remainder of their lives. However, there is now an effective treatment using the nucleoside analogue drug ribavirin combined with interferon<ref>Witthoft T, Moller B, Wiedmann KH, Mauss S, Link R, Lohmeyer J, Lafrenz M,Gelbmann CM, Huppe D, Niederau C, Alshuth U. Safety, tolerability and efficacy of peginterferon alpha-2a and ribavirin in chronic hepatitis C in clinical practice: The German Open Safety Trial. J Viral Hepat. 2007 Nov;14(11):788-96.</ref> The treatment of chronic carriers of the Hepatitis B virus by using a similar strategy using lamivudine is being developed.<ref>Rudin D, Shah SM, Kiss A, Wetz RV, Sottile VM. Interferon and lamivudine vs. interferon for hepatitis B e antigen-positive hepatitis B treatment: meta-analysis of randomized controlled trials.Liver Int. 2007 Nov;27(9):1185-93.</ref>

Applications

Life sciences and Medicine

Viruses are important to the study of molecular and cellular biology as they provide simple systems that can be used to manipulate and investigate the functions of cells. The study and use of viruses have provided valuable information about aspects of cell biology. For example, viruses been useful in the study of genetics and helped our understanding of the basic mechanisms of molecular genetics, such as DNA replication, transcription, RNA processing, translation, protein transport, and immunology.

Geneticists often use viruses as vectors to introduce genes into cells that they are studying. This is useful for making the cell produce a foreign substance, or to study the effect of introducing a new gene into the genome. In similar fashion, virotherapy uses viruses as vectors to treat various diseases, as they can specifically target cells and DNA. It shows promising use in the treatment of cancer and in gene therapy.

Eastern European scientists have used phage therapy as an alternative to antibiotics for some time and interest in this approach is increasing, due to the high level of antibiotic resistance now found in some pathogenic bacteria.<ref name="pmid16258815">Modèle:Cite journal</ref>

Materials science and nanotechnology

Current trends in nanotechnology promise to make much more versatile use of viruses. From the viewpoint of a materials scientist, viruses can be regarded as organic nanoparticles.<ref> Proceedings of SPIE -- Volume 6413Smart Materials IV, Nicolas H. Voelcker, Editor, 64130F (Dec. 22, 2006). Hybrid organic-inorganic nanoparticles: controlled incorporation of gold nanoparticles into virus-like particles and application in surface-enhanced Raman spectroscopy Marcus Niebert, James Riches, Mark Howes, Charles Ferguson, Robert G. Parton, Anton P. J. Middelberg, Llew Rintoul, and Peter M. Fredericks.Queensland Univ. of Technology (Australia) (published online Dec. 22, 2006)</ref> Their surface carries specific tools designed to cross the barriers of their host cells. The size and shape of viruses, and the number and nature of the functional groups on their surface, is precisely defined. As such, viruses are commonly used in materials science as scaffolds for covalently linked surface modifications. A particular quality of viruses is that they can be tailored by directed evolution. The powerful techniques developed by life sciences are becoming the basis of engineering approaches towards nanomaterials, opening a wide range of applications far beyond biology and medicine.<ref name=fischlechner>Modèle:Cite journal</ref>

In April 2006 scientists at the Massachusetts Institute of Technology (MIT) created nanoscale metallic wires using a genetically-modified virus.<ref name="mitvirusbattery"> Researchers build tiny batteries with viruses

. MIT News Office  
 

 

. Retrieved on 2007-04-05. </ref> The MIT team was able to use the virus to create a working battery with an energy density up to three times more than current materials. The potential exists for this technology to be used in liquid crystals, solar cells, fuel cells, and other electronics in the future.

Weapons

Modèle:Details The ability of viruses to cause devastating epidemics in human societies has led to the concern that viruses could be weaponized for biological warfare. Further concern was raised by the successful recreation of the infamous 1918 influenza virus in a laboratory.<ref name="cdcnews"> Researchers Reconstruct 1918 Pandemic Influenza Virus; Effort Designed to Advance Preparedness

. Centers for Disease Control  
 

 

. Retrieved on 2007-04-05. </ref> The smallpox virus devastated numerous societies throughout history before its eradication. It currently exists in several secure laboratories in the world, and fears that it may be used as a weapon are not totally unfounded. The vaccine for smallpox is not safe and during the years before the eradication of smallpox disease more people became seriously ill as a result of vaccination than did people from smallpox.<ref name="pmid12911836">Modèle:Cite journal</ref> and smallpox vaccination is no longer practiced.<ref name="pmid15578369">Modèle:Cite journal</ref> Thus, the modern global human population has almost no established resistance to smallpox; if it were to be released, a massive loss of life could be sustained before the virus is brought under control.

See also

• Influenza
• Rotavirus
• Herpes simplex virus
• Hepatitis B virus
• Satellite

Electron micrographs of viruses

Norovirus. This RNA virus causes winter vomiting disease. It is often in the news as a cause of gastro-enteritis on cruise ships and in hospitals.	Caliciviruses are related to Noroviruses.	Torovirus. An enveloped RNA virus.	Coronaviruses are a group of viruses that have a halo, or crown-like (corona) appearance when viewed under a microscope.
Ebola Virus is a filamentous RNA virus.	Measles virus. This is called a thin section where the virus particle has been cut in two.	Respiratory Syncytial Virus (RSV). In this preparation the ribonucleoprotein can be seen as a herring bone pattern.	Parvovirus B19. Parvovirus B19 is a small DNA virus best known for causing a childhood exanthema called fifth disease or erythema infectiosum.
Human Papilloma Virus	Influenza virus	Transmission electron micrograph of Herpes virus an enveloped virus that looks like fried eggs by negative stain electron microscopy.	TEM micrograph of Poliovirus virions.

References

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