Alex Scoble
says, "Douglas Schweitzer prefers Sophos for the Mac, which is not free to the general public and free Avast! 4 Home Edition for the PC. Now is a good time to mention that a lot of Mac users, unwisely, use no antivirus protection at all, which is a recipe for future disaster, unless you don't buy the hype that Apple will soon rule the world...again."
While I think someone could write a virus for the Mac, the only threat are Word macro viruses. There are free virus scanners for the Mac such as
ClamXav 1.0.1. As they says on their homepage, " Back in the days before OS X, the number of viruses which attacked Macintosh users totalled somewhere between about 60 and 80. Today, the number of viruses actively attacking OS X users is...NONE! However, this doesn't mean we should get complacent about checking incoming email attachments or web downloads, for two reasons. Firstly, there's no guarantee that we Mac users will continue to enjoy the status quo..."
My experience are these products are more trouble than they are worth. OS X is unix and like Linux and other *nix out there, there are currently no know viruses or adware out there. I'm more worried about the threat of root-kits, which patch the kernal than I am viruses.
My advice is run a free virus scanner if you are really worried about it, but I don't think there is much to worry about.