This article is from the Computer viruses FAQ, by David Harley D.Harley@icrf.icnet.uk, George Wenzel gwenzel@telusplanet.net and Bruce Burrell bpb@umich.edu with numerous contributions by others.
Many of the most widely-spread viruses are Boot Sector Infectors,
which can't normally infect over a serial or network connection.
Writers of shareware, freeware etc. are no more prone to accidental
infection than commercial publishers, and possibly less. The only
'safe' PC is still in it's original wrapping (which doesn't mean
it isn't already infected...) And don't forget that shrinkwrapped
software may have been rewrapped.
As well, the most common viruses today are macro viruses, which depend
on you running a commercial application (usually MS Word or Excel).
They spread via documents exchanged between computers, which is a common
occurrance on many systems, regardless of how 'connected' they are.
 
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