It is not possible to give an exact number because new viruses are being created literally every day. Furthermore, different anti-virus researchers use different criteria to decide whether two viruses are different or one and the same. Some count viruses as different if they differ by at least one bit in their non-variable code. Others group the viruses in families and do not count the closely related variants in one family as different viruses.
Taking a rough average, as of October 1992 there were about 1,800 IBM PC viruses, about 150 Amiga viruses, about 30 Macintosh viruses, about a dozen Acorn Archimedes viruses, several Atari ST viruses, and a few Apple II viruses.
However, very few of the existing viruses are widespread. For instance, only about three dozen of the known IBM PC viruses are causing most of the reported infections.
How do viruses spread so quickly?
This is a very complex issue. Most viruses don't spread very quickly. Those that do spread widely are able to do so for a variety of reasons. A large target population (i.e., millions of compatible computers) helps... A large virus population helps... Vendors whose quality assurance mechanisms rely on, for example, outdated scanners help... Users who gratuitously insert new software into their systems without making any attempt to test for viruses help... All of these things are factors.
What is the plural of "virus"? "Viruses" or "viri" or "virii" or...
The correct English plural of "virus" is "viruses." The Latin word is a mass noun (like "air"), and there is no correct Latin plural. Please use "viruses," and if people use other forms, please don't use VIRUS-L/comp.virus to correct them.
When reporting a virus infection (and looking for assistance), what information should be included?
People frequently post messages to VIRUS-L/comp.virus requesting assistance on a suspected virus problem. Quite often, the information supplied is not sufficient for the various experts on the list to be able to help out. Also note that any such assistance from members of the list is provided on a volunteer basis; be grateful for any help received. Try to provide the following information in your requests for assistance:
This is a difficult question to answer. Antiviral software is a kind of insurance, and these type of calculations are difficult.
There are two things to watch out for here: the general "style" of the software, and the signatures which scanners use to identify viruses. Scanners should be updated more frequently than other software, and it is probably a good idea to update your set of signatures at least once every two months.
Some antiviral software looks for changes to programs or specific types of viral "activity," and these programs generally claim to be good for "all current and future viral programs." However, even these programs cannot guarantee to protect against all future viruses, and should probably be upgraded once per year.
Of course, not every anti-virus product is effective against all viruses, even if upgraded regularly. Thus, do *not* depend on the fact that you have upgraded your product recently as a guarantee that your system is free of viruses!