There are linux viruses, but the're very rare and usually they don't have a dangerous payload. Worms are more common, but again they are pretty rare as well. If you keep up with your security patches and have a firewall, you're probably OK.
If you're hyper-paranoid, there are commercial anti-virus products made by Kaspersky Labs and F-Prot. I think Symantec may have a linux version of NAV, but I'm not sure.