The JargonFile starts:
These days, virus has come to mean any trojan horse, script or other 'nasty' program that has the ability to self replicate, and cause problems.
Thankfully, due to the security model of Unix and clones and the fact that MicrosoftWindows is (a) so much easier to target (b) running on 100x as many machines as Linux, there aren't many Linux viruses out there.
One thing Linux is good at though, is stopping and dealing with Windows viruses. You can do what you like with them with almost no danger of infection1?.
ClamAV is a Free Anti-Virus program for Linux. See MailScannerNotes for information about using Linux to protect your Windows clients from email viruses.