A virus is introduced onto your computer through a previously infected program. Most commonly the virus is introduced by trading Public Domain programs, usually games.
It is also possible to download an infected application from a bulletin board, although most programs are checked by the System Operator of the bulletin board before they are made public. Unfortunately all System Operators are not so conscientious.
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Also there have been a few (very few) know cases of a software publisher sending out a master disk with a virus on it! So even if you only buy shrink wrap software it is possible to get a virus.