There aren't any (as in zero) Mac OS X viruses. There are several pieces of malware available, but they're not able to work by themselves.
Most Mac users are really stupid about security, though, but just about the same percentage as Windows users. In the past three years, even more Mac users are stupid about security since a lot of newer Mac users came from Windows.
The biggest problem is that some users will click on anything and there is a trojan horse attack for every system, especially after you tell them not to do it.
How many people are still running Internet Explorer 6? You might as well write down your personal information and hand it to everyone you see because some people would refuse to take the paper and you'd be safer than using IE 6.