Again, this has been disproven time and again. It's a canard.
The fact is there are NO viruses active for Mac OSX (I do remember the WDEF virus in 1990 or so, but that's dead). There are THOUSANDS active TODAY for Windows.
But it doesn't really matter, does it? You have to spend your time and money supporting an industry that does nothing (your virus / anti-virus industry), I don't. Whatever the reasons you think that's OK, it's still a fact.
Oooh! Oooh!
The Autostart Worm; don't forget the Autostart Worm.
That was circa 1996 or thereabouts, and I had to (gasp) DELETE A FILE! Curses!
On the serious side, I think the windoze guys do have (kind of) a point. The greater market share means a tremendously increased chance of people who haven't heard of Norton AntiVirus or any competitor thereof.
One of my best friends, for instance, runs his business off his home office setup. He calls me for tech support. I told him I might be able to lend a hand w/hardware and productivity/utility software, but that as a Mac Guy, if he needed OS support, I was NOT his guy.
Well, to make a long story short, there were no hardware failures to cause his problem (hard disk shutting down w/o warning). The disk was pretty new, so the odds of a lemon, I thought, were long.
Had him buy AntiVirus by Symantec. He ran the program. Great day in the morning, that man had several HUNDRED virii floating around on his computer. He had NEVER run antivirus software, he had a constant internet connection, etc.
Two years. On a Wintel platform. NEVER ran antivirus.
THAT, IMHO, is a BIG part of why the hackers like the Wintel platform. Lots of people who want the cheapest, and have little or no clue about anything beyond business/games. It's a digital petri dish.