There is no need for anti virus for MAC. MAC OSX is built differently from Windows. The inner core is more like linux, it uses priviligies when running programs. A program can't fully take over your machine if you don't give it priviligies to do so, unlike Windows older than Vista where everybody ran a program under a Administrator account, which gave every programs full priviligies to do what they want. In Vista and W7, Microsoft changed that to run programs in restricted priviligies in a virtual environment.
So if you run a virus on a MAC, it won't infect and hurt your computer to a slight degree that you require a full reformat/reinstallation, if the virus does not know about any security holes which I think is probably none now. The chance of getting a virus infection on a MAC is pretty slim.
The best prevention for viruses on a MAC is to ALWAYS update OSX. Apple has probably fixed security issues in new updates.