F1) How many viruses are there?

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.


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