That's problem. Regular people don't realize (along with many peopler on Internet that claim to be experts) that OS X has no viruses in the wild. Most Mac users get Trojan emails that sucker them (usually from less than reputable sites) or Java Exploits or warze and give their password tom install without thinking.
The Antivirus claims for OS X just scan your Mac for Windows exploits in emails you receive so you don't pass them along to another Windows using coworker/relative.
If you really want a virus checker that is free for OS X
ClamAV.
IMHO you are better off with the reverse firewall shareware
Little Snitch. You will be surprised how many programs connect to other services that they shouldn't.
