First: True, there are no known virus' in the wild for OS X. Secondly, even if there were, you'd first have to give said fabled virus permission to run on your machine. This is usually accomplished by typing in your administrative password, associated with installing something or letting a script run.
Thirdly, if you don't surf carelessly or visit nasty places or download dangerous things on the intertubes, then technically, you shouldn't be worried about any of that stuff, even on a Windows machine! As far as malware and spyware goes, in the three years of owning my MacBook Pro's, I've yet to run into anything. I've also never had the need to run any such program associated with these things.
Ah, the good ol' Windows days, where half my time was wasted away for such tripe. (and I ain't talkin cow innards bub)
Doug