Well they do have a lot of Antivirus for Macintosh too. But did Mac ever get a virus that may harm the OS? The answer is NO..There are a few trojan and virus but No never going to effect the OS, why? because the OS is very strictly protected via software update.
Unix and Linux is very secure and hardly anyone would want to write a code to destroy the OS like they did on windows.

Why because everyone love Linux OS , well Mac by far one of the secure none free OS out there. Windows has a lot of haters and loop holes.

Linux loop hole will be fix in the next update, this is the point of OpenSource. If one found it, they can contribute and stop it.