Well, not entirely true - there have been a couple Mac worms hidden in pirated versions of software.
Just to clarify, there are no known viruses that exist in the wild that affect OSX
There are however, a few well publicized trojans that travelled with some pirated software (Adobe's Creative Suite and iWork) and through some porn downloads
The simple difference is, viruses require no user input to execute
Trojans, on the other hand, require the user to install them by using their administrative password
The best anti virus for the Mac and OSX is safe computing practices and common sense. Only download software from reputable sources. Know what you are downloading and from where, and know what you are installing when you enter your administrative password.
OSX can carry and transmit Windows viruses, but they do not affect the Mac
Viruses for Mac OSX DO exist, theres just not anywhere near as many. Over the last 10 years Macs have been getting more poplular so people writing viruses will start to target the Mac world too.
Iwork 2009 had viruses attached to the files. Also Bit torrent files can have viruses. Last reported was also 2009. Check out the link i post above. Most recent is 2009.
I'm late to the party on this one they CAN get them but its very rare and I have never gotten one in the 5 months I have owned my Mac.