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Ehrenkater (talk | contribs) Undid revision 1299963835 by Graham Beards (talk) While "size" is ambiguous, this would normally be expected to refer to the length rather than the width. Hence the array of viruses would need to be lengthwise. |
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A '''virus''' is a tiny [[infectious agent]] that [[reproduction|reproduces]] inside the [[Cell (biology)|cells]] of living [[Host (biology)|hosts]]. When infected, the host cell is forced to rapidly produce thousands of identical copies of the original virus. Unlike most [[Organism|living things]], viruses do not have cells that divide; new viruses assemble in the infected host cell. But unlike simpler infectious agents like [[prion]]s, they contain [[Introduction to genetics|genes]], which allow them to [[Mutation|mutate]] and evolve. Over 4,800 [[List of virus species|species of viruses]] have been [[List of virus taxa|described in detail]]<ref name="pmid29754305">{{cite journal |vauthors=King AM, Lefkowitz EJ, Mushegian AR, Adams MJ, Dutilh BE, Gorbalenya AE, Harrach B, Harrison RL, Junglen S, Knowles NJ, Kropinski AM, Krupovic M, Kuhn JH, Nibert ML, Rubino L, Sabanadzovic S, Sanfaçon H, Siddell SG, Simmonds P, Varsani A, Zerbini FM, Davison AJ |s2cid=21670772 |title=Changes to taxonomy and the International Code of Virus Classification and Nomenclature ratified by the International Committee Taxonomy of Viruses (2018) |journal=Archives of Virology |volume=163 |issue=9 |date=September 2018 |page=2601 <!-- CITATION BOT DO NOT CHANGE THIS ... citation is specific to this page --> |pmid=29754305 |doi=10.1007/s00705-018-3847-1 |url=https://hal-pasteur.archives-ouvertes.fr/pasteur-01977332/file/King2018_Article_ChangesToTaxonomyAndTheInterna.pdf}}</ref> out of the millions in the environment. Their origin is unclear: some may have [[evolution|evolved]] from [[plasmid]]s—pieces of DNA that can move between cells—while others may have evolved from [[bacteria]].
Viruses are made of either two or three parts. All include [[gene]]s. These genes contain the encoded biological information of the virus and are built from either [[DNA]] or [[RNA]]. All viruses are also covered with a [[protein]] coat to protect the genes. Some viruses may also have an [[viral envelope|envelope]] of [[Lipid|fat-like substance]] that covers the protein coat, and makes them vulnerable to soap. A virus with this "viral envelope" uses it—along with specific [[Cell surface receptor|receptors]]—to enter a new host cell. Viruses vary in shape from the simple [[tobacco mosaic virus|helical]] and [[icosahedron|icosahedral]] to more [[bacteriophage|complex]] structures. Viruses range in size from 20 to 300 [[nanometre]]s; it would take 33,000 to 500,000 of them,
Viruses spread in many ways. Although many are very specific about which host species or [[Tissue (biology)|tissue]] they attack, each [[species]] of virus relies on a particular method to copy itself. [[Plant pathology#Viruses, viroids and virus-like organisms|Plant viruses]] are often spread from plant to plant by insects and other [[organism]]s, known as ''[[Vector (epidemiology)|vectors]]''. Some [[Virus#Role in human disease|viruses of humans]] and other animals are spread by exposure to infected bodily fluids. Viruses such as [[influenza]] are spread through the air by droplets of moisture when people cough or sneeze. Viruses such as [[norovirus]] are transmitted by the [[fecal–oral route|faecal–oral route]], which involves the contamination of hands, food and water. [[Rotavirus]] is often spread by direct contact with infected children. The human immunodeficiency virus, [[HIV]], is transmitted by bodily fluids transferred during sex. Others, such as the [[dengue virus]], are spread by [[Hematophagy|blood-sucking insects]].
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