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Viruses are obligate intracellular parasites. They are incapable of replicating outside a host cell.

Viruses cause many infectious diseases, and may also be carcinogenic.[1]

It should be noted that some human viruses cause subclinical infection, or may be asymptomatic in immunocompetent hosts e.g. GBV-C, TTV.

Almost all viruses are too small to be visualised by light microscopy. Virus size is measured in nanometres (nm). They can only be visualised by electron microscopy. The one exception is the recently discovered mimivirus, which is large enough to be discernible by light microscopy.

Viruses can be classifed according to whether the genetic material they contain is DNA or RNA, how many strands of DNA they contain and whether the virus is enveloped or not.

A single virus particle is known as a virion.

Probably descended from us, rather than vice versa[2]. Nothing simpler looks like life, and this is marginal until you get to the larger DNA viruses[3].

Usually RNA ones mutate faster than DNA ones[4], Herpes group viruses such as Varicella bind themselves into our cells and recrudesce much later, and retroviruses are written back from their RNA into DNA using our cellular machinery[5].

We lack specific treatments for most, exceptions being aciclovir for Herpes and Varicella, cidofovir, the lipophilic nucleotide analog formed by covalently linking cidofovir with 3-(hexdecyloxy)propan-1-ol(CMX001) or interferon-β for smallpox, amantadine and oseltamivir for influenza A, and several drugs for the HIV.

Classification of viruses

See International Committee on Taxonomy of Viruses (ICTV):

See also



This article is a work in progress. Please feel free to contribute to it.