Actually, I'll tell you a cute story. Apple often gives their customers free software, and recently they added to the list a comprehensive back-up software and an anti-virus software. I downloaded both, and decided to run the anti-virus software. This was the first time any such program has been run on my Mac since I installed OS X last October, and there were no viruses at all! I really expected there to be something floating around somewhere, but there wasn't.
Take care,
~Chris