Well, Mac OS was originally designed with a closed framework so it was nearly impossible to get into. Because of this Apple decided not to include any main security features (besides basics) with OS X And I don't think that there is any aftermarket virus software for Macintosh.
People found ways to get around it (Hell RTS.D Backdoor, google it) and now Apple is basically going to go into panic mode.
WHAT NOW, STEVE?! WHAT NOW!?!?