The compilation of a unified list of computer viruses is made difficult because of naming. To aid the fight against computer viruses and other types of malicious software, many security advisory organizations and developers of anti-virus software compile and publish lists of viruses. When a new virus appears, the rush begins to identify and understand it as well as develop appropriate counter-measures to stop its propagation. Along the way, a name is attached to the virus. As the developers of anti-virus software compete partly based on how quickly they react to the new threat, they usually study and name the viruses independently. By the time the virus is identified, many names denote the same virus.
Another source of ambiguity in names is that sometimes a virus initially identified as a completely new virus is found to be a variation of an earlier known virus, in which cases, it is often renamed. For example, the second variation of the Sobig worm was initially called "Palyh" but later renamed "Sobig.b". Again, depending on how quickly this happens, the old name may persist.
In terms of scope, there are two major variants: the list of "in-the-wild" viruses, which list viruses in active circulation, and lists of all known viruses, which also contain viruses believed not to be in active circulation (also called "zoo viruses"). The sizes are vastly different: in-the-wild lists contain a hundred viruses but full lists contain tens of thousands.