Well, you have to be pretty darn stoopid to catch this; first you need to go to a shady website (many of them are illegal download sites, .nu domain), and secondly, you have to enter your master password when the website asks for it...
But, never underestimate the amount of human stupidity - just look around... According to this, over half a million Macs have been affected:
(Oh, BTW, Bob, if you happen to read this: It isn't a virus, it's a Trojan. Big difference - a virus infects without your approval... )