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=== Size ===
[[File:Virus size.png|right|thumb|Virions of some of the most common human viruses with their relative size. The nucleic acids are not to scale.]]
Viruses are among the smallest infectious agents, and are too small to be seen by [[Optical microscope|light microscopy]]; most of them can only be seen by [[electron microscopy]]. Their sizes range from 20 to 300 [[nanometre]]s; it would take 30,000 to 500,000 of them, laid end to end, to stretch to one centimetre (0.4 in).<ref name="Topley-and-Wilson33-55" >{{harvnb|Collier|Balows|Sussman|1998|pp=33–55}}</ref> In comparison, bacteria are typically around 1000 nanometres (1 [[micrometre|micrometer]]) in diameter, and host cells of higher organisms are typically a few tens of micrometers. Some viruses, such as [[megavirus]]es and [[pandoravirus]]es, are relatively large viruses. At around 1000 nanometres, these viruses, which infect [[amoeba]]e, were discovered in 2003 and 2013.<ref>{{cite journal |vauthors=Abergel C, Legendre M, Claverie JM |title=The rapidly expanding universe of giant viruses: Mimivirus, Pandoravirus, Pithovirus and Mollivirus |journal=FEMS Microbiol. Rev. |volume=39 |issue=6 |pages=779–796 |date=November 2015 |pmid=26391910 |doi=10.1093/femsre/fuv037 |doi-access=free }}</ref><ref>{{cite journal |vauthors=Philippe N, Legendre M, Doutre G, Couté Y, Poirot O, Lescot M, Arslan D, Seltzer V, Bertaux L, Bruley C, Garin J, Claverie JM, Abergel C |s2cid=16877147 |title=Pandoraviruses: amoeba viruses with genomes up to 2.5 Mb reaching that of parasitic eukaryotes |journal=Science |volume=341 |issue=6143 |pages=281–286 |date=July 2013 |pmid=23869018 |doi=10.1126/science.1239181 |bibcode=2013Sci...341..281P |url=https://hal-cea.archives-ouvertes.fr/cea-00862677/file/phi.pdf }}</ref> They are around ten times wider (and thus a thousand times larger in volume) than [[influenza virus]]es, and the discovery of these "giant" viruses astonished scientists.<ref name="Zimmer">{{cite news | url=https://www.nytimes.com/2013/07/18/science/changing-view-on-viruses-not-so-small-after-all.html | title=Changing View on Viruses: Not So Small After All | work=The New York Times | date=18 July 2013 | access-date=20 December 2014 | vauthors = Zimmer C}}</ref>
=== Genes ===
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