What is a computer virus? It's a small program designed to copy itself onto
your hard disk and run surreptitiously. There's usually no way to tell it's
there unless you check for it specifically, or unless it causes problems or
becomes a nuisance.

Most of the viruses that have been observed on Macintosh computers have not
been too evil. They do things like causing your computer to beep or behave
oddly, and actual harm (like losing files) is unusual. They have not yet
become a major problem in the Macintosh community at the University of
Washington. However, they do exist here and you should know how to protect
your computer against them.

One of the better defenses, if you feel like paying money, is Symantec
Anti-virus for Macintosh, or "SAM". This package maintains an active search
for viruses and suspicious activity any time you use a floppy disk or install
a new piece of software. It's available for $50 or so through any of the
mail-order software catalogs.

A free alternative, and an educational one, is a program available from the UW
Computing & Communications FTP server called Disinfectant. It was written by
John Norstad at Northwestern University, and is kept up-to-date with regular
distribution of new versions. If you download this software and run it, it
will check for any viruses on your hard disk and remove them; it will also
show you a list of all the viruses it knows about and tell you where they came
from and what they do. It isn't as thorough as SAM (you have to manually start
the program and tell it to check a floppy disk, if you want to do that) but,
again, it's free. If you just want to check your hard disk out occasionally,
like I do, this is a good tool.

Whether you use any virus detection tools or not, you should always exercise
common sense -- don't use any floppy disk or run any program if you don't know
where it came from or what it's supposed to do. To minimize the chances of a
virus affecting your computer, use only prepackaged software obtained from a
reputable distributor and don't trade floppy disks among different computers
any more than you have to.