A computer virus is, in essence, a program. However it is camouflaged,
hidden inside another program, disguised as a "freebie" which is "too good
to miss", etc., it must be run in order to work its mischief.

You can only be attacked by a virus written specifically for the operating
system on your computer, so, say, an IBM compatible virus can't attack,
say, a Mac.

It is IMPOSSIBLE to catch a computer virus just by downloading a file from
a Web site, or by, say, reading your mail.  Any downloaded file must be run
for a computer virus to have any effect.

Having said that, I must add two warnings:

1. There is a type of "virus" which is going the rounds at the moment, that
is in the form of a macro for Microsoft Word documents. Is this a program?
In effect, yes, since it triggers a program module (In Word v.6 or Word
95), or runs a Visual Basic for Applications program stub (in Word 97).

However, neither of these macro types will run if you view the text file
containing them in a program OTHER THAN Microsoft Word (say, Notepad or
Write, or even the Microsoft Word Viewer). And this I recommend you do, if
you find a Word file (usually with the suffix .DOC) in downloaded material.

Microsoft have come out with a program that will warn you if a document
contains a suspect macro, but on the principle of "belt AND braces", I
believe it is better not to tempt fate, and to use a program other than
Word to view document files.  Incidentally, if you use Write, it will
format the text in the same way as Word would, but it cannot use macros.

2. A virus may be attached to a downloaded file that is stored in a
self-extracting compressed file format, such as a PKZIP file with an EXE
suffix.  In this case, just running the self-extractor *could* activate any
virus attached to it.

It is better, if you can, to open the self extracting file in a utility
program like WinZip, where you can examine any document files, and you can
use a virus scanning program on executable files, including the
self-extracting file itself.

But above all, one must be wary of virus attacks at all times:
1. Use an anti-virus program
2. Use an anti-virus scanner on downloaded files.
3. Always run a new program on a computer other than your main one if
possible, or on a sacrificial floppy disk or partition.
4. Make all your system files read-only.
5. And, most important of all, keep a recent backup, in case all else
fails.

Having said all that, I must say that the only place I have seen a computer
virus in 20 years of PC computing, is in a computer lab, and I have never
been attacked by one, so I must be living right. <grin>

Jim