There are currently no viruses that affect Mac OS X.
That's not to say that there will never be any, but at present you don't have to worry about malware infecting your system.
Anti-virus programs are available; they'll delete any PC viruses you get and may come in handy if a successful OS X virus ever does come out. (Virex is free with a .Mac account, and other programs can be found on VersionTracker
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