Let's consider a windows system first. An attachment could contain a
virus which we all know. However, a virus can also be contained in an
html email but not a plain text email. In the last 2/3 years it seems
that providers do scan on the server. I've tested the ones i use with a
test-virus [netscape, inbox.com, Gmail] and found that to be the case.
It would seem that the days of a virus being sent around via addresses
in one's addressbook are over.

As for Linux - fewer people use it so less likely to have a virus that
can do damage... but i'd install an antivirus package like Avast for
Linux or Clamwin. The virus's mostly are designed for windoze apps... so
instead of Word you're likely to have OpenOffice - so less likely to do
damage.

I'm no great authority but i haven't had a virus for 5 years. Perhaps
someone else more knowledgeable would like to comment.