There are not many OS-X viruses in the wild, yet. But even though OS-X viruses aren't as common as Windows viruses by a large margin, they are still out there -- and they're multiplying.
Wrong.
There are many forms of Mac security breaches. Mac users aren't immune to problems. But they tend to come in the form of social-engineering hacking, buffer overflow exploits, trojans, and so on.
You're probably confused because the media often calls all of these things "viruses" when they're actually not.
They're still bad things, and Mac users can still get hurt by them, but your statement that OS X viruses are multiplying is a false one.
If you don't believe me, here's a test for you: Try and find me a report of a Mac virus that's not a proof-of-concept or a trojan mislabeled as a virus by an ignorant reporter. I don't think you'll be coming up with a whole lot.
EDIT: Actually, now that I think about it, that article is BOTH of those things. The exploit mentioned in it didn't do any harm, so it's a proof-of-concept. And even if it did do harm, it was a program that had to be installed by the user, which makes it a trojan, not a virus.