Macs (and Unix) do get viruses (or viri, if you prefer).
Mac systems are less often attacked, as pointed out above, because they're not the "large target," and you seldom hear about Mac infections just because it's hard to sensationalize something that affects so few users. (And please, I don't mean any condescension.)
Those who attack Unix systems usually don't bother with individual users, for the same reason that Mac's get ignored, and most "large base" users don't publicize it because (1) they usually have the tech resources to handle it "in-house," and (2) it's bad for business to admit it publicly.