I've had Macs for 16 years, and started on the net with BBSs, using my first Mac, an LC, running System 7.x. Neither it nor any of the other machines — more specifically, their Mac systems (I ran Windows under Virtual PC), I've had since have been compromised.
I used a Mac anti-virus program running in the background (pre OS X) for a couple of months in '94 or '95, then trashed it. But using free AV apps, I occasionally would check for viruses but never found one.
Now with OS X (at the moment, 10.4.9) on a dual 1-GHz G4, I use the firewall along with its two "advanced" settings, but I hardly ever check for Windows viruses in emails on the premise that Windows users can dig and flood their own moats.
Five or six years ago, running System 9.2 on the Mac side of a G3, I ran Windows '95 under Virtual PC for a while because I had to learn more about Windows than simply running it, and almost immediately experienced the joy of entertaining uninvited guests. After a few months I killed Windows permanently.
In 16 years, only Windows viruses have tried to take up residence, and none of them, of course, could do anything to the Mac OS.