Have a read of this:
http://www.linux.com/feature/60208
Strictly speaking there may be the odd virus, but last time I looked I believe there was only a few and mostly only existed in the lab.
(Talking about viruses that affect Linux)
If you build a server/gateway that service MS systems, there are pkgs that will scan for MS viruses, but those cannot run natively on Linux.
See also the Security forum.
Edit: 'those cannot run natively on Linux' => 'MS viruses cannot run natively on Linux.'
Just to clarify