Unlike Luca for cellular organism (14
), there is no presumed common ancestor for viruses (12
). Therefore current virus classification comprises seven independent classes, according to the Baltimore system (4
). This classification is based on the nature of the nucleic acids in the virion particle: dsDNA, ssDNA, dsRNA, ss(+)RNA, ss(−)RNA, ssRNA(RT) or ssDNA(RT).
Virus abundance on earth is higher than initially expected and recent studies have unveiled millions of viruses per millilitre of seawater and billions per cubic centimeter in nearshore surface sediments (24
); most of them are unidentified. As virus discovery accelerates, virus taxonomy has to be modified and completed each year (). In ViralZone, the starting point to access virus fact sheets are the seven Baltimore taxonomic pages (4
) containing the whole list of known virus families and genera (A). This list is reviewed each year as new viruses are constantly being described (8
). The advantage of a website is that it can be incrementally updated while it can take years to publish new reference books. For example, the International Committee on Taxonomy of Viruses (ICTV) published important taxonomic changes on August 2009 on its website and the ViralZone taxonomy was updated accordingly only one month later.
The growing number of virus taxons
Figure 2. (A) ViralZone Baltimore Index. (B) Taxonomic index for ssRNA(+) viruses, classified by order, family then genus. Colour spots indicate the host infected by each virus genera: pink for human and other vertebrates, purple for non-human vertebrates, green (more ...)
From a public health point of view, providing comprehensive knowledge for all known virus genera turns out to be extremely useful when a new pathogen emerges out of a neglected virus family. A recent example is provided by the Xenotropic Moloney murine leukaemia virus-Related Virus (XMRV), which has recently raised the interest of the scientific community for its potential involvement in prostate cancer (22
) and/or chronic fatigue syndrome (18
). Since specific gammaretrovirus resources on the web were scarce, a direct consequence has been a dramatic increase in the number of hits to the corresponding ViralZone page (www.expasy.org/viralzone/all_by_species/67.html
) that reached close to 2000 visitors in November–December 2009 (source: Google Analytics).