That's a pretty strong statement there. What about the recent Botnet trojan that was found inside cracked versions of Photoshop for Mac & iWork 09?
As dtravis7 said, there is a distinct difference between a virus and a trojan. Yes, trojans exist on all platforms. Heck, one can simply make a malicious script and call it "BESTEST PROGRAM EVER, CLICK MEEEEEEE!!!" and it would qualify as a trojan. With even a modicum of common sense, most users will never experience a trojan on OS X. Fortunately, 10.6 included measures which block even these few trojans from running without stern warnings.
There's also the OSX/Inqtana.A, a Java-based worm that exploits a vulnerability in the Bluetooth file and object exchange services in Mac OS X 10.4.
Again, a worm is not a virus. A worm is an exploit for a vulnerability, and in this case, it's one that has long since been patched.
Lots of other info out there about Mac viruses that go all the way back to the beginnings of OSX that counter the false idea that there are no OSX viruses.
Find a true, self-replicating, self-propagating virus and I will grant you that assertion.
I had a CIS PhD almost laugh me out of the room at a lab meeting for suggesting that same notion just recently. Oops! Wasn't that embarrassing now? They do exist and are out there, they are just not as common as PC viruses but they are getting there...
Somehow I doubt that. There are literally thousands of viruses in the wild for Windows. If we include inactive viruses, that would be hundreds of thousands. I'm not going to say that it will never get there, but it's pretty unlikely.
DOS-based and NT-based Windows versions operate in a completely different way from UNIX-based OSes. Until Microsoft implements a truly capable Discretionary Access Control mechanism (and UAC is a good start, but weakened by provisions to keep legacy software running), it will be a very long time before we'll see the kind of resiliency that we see from OS X.
"According to another online security company, Intego, as of January 2009, over 20,000 Mac users were known to have their computers infected with the Botnet trojan or other viruses. This, of course, is just a fraction of the number of PCs infected with malware; but it represents a sharp rise in the number of Macs affected over the past two years."
Keep yer interwebz safe y'all.
If your primary business is convincing the sheep that they need to be kept in the pen, how do you market your product? Scare tactics. Some of the claims from AV peddlers are not just merely untrue, they are irresponsible. But hey, if you're the kind of person that believes everything a used car salesman says, then so be it.