certainly the relative size of the Mac community helps to protect them somewhat. Back in the 90's when the number of Macs out there was smaller than it is today there were Mac viruses. I even caught one once by bringing home a questionable copy of some software from work...
at this moment in time (and since the release of OSX however many years ago that was) there are no circulating Mac viruses. There are several Mac virus scanner programs and all they do is look for the old 90's viruses in your classic emulation environment (for running software from the 90's
) and scan your email for all the worm and trojan programs so that you dont inadvertently forward them to your windows user friends.
There have been potential security issues. There was a buffer overflow found in one of the rendering library, same as in windows and unix all using the same code I think, but it was patched quickly enough and no examples of anyone actually using it were ever documented. I'm sure there will be more such things in the future.
Once virus remover company did write a proof of concept virus for OSX just to prove that their software wasn't completely useless. But to install it you had to run their program and enter your root password.
Just cause you run OSX doesn't mean you're exempt from practicing good internet hygiene. If you click on and try to run everything that you get in your email basket eventually one of them will be a mac trojan program. But if you can keep from doing anything like that, the opportunity for software to get into your Mac without you doing anything is so close to nil as to make no difference.
I own no virus checkers, I run no popup or adware blockers, I do not scan regularly for keystroke recorders. These things simply don't waste my time during the day. I spend all day every day online with this machine, I run Mac servers. I know the importance of keeping them up to date with security patches and such, but if you do that you simply will not have any problems.
The counter argument that folks so like to bring up is that well, if the Mac gets more popular then there will be viruses for it too. Thats an arguable point since as I mentioned above, the permissions and such are different on the Mac and getting anything really interesting running requires your root password which you would have to enter for it. And it's besides the point anyway. Even if it were true that there will be a Mac virus in 5 or 10 years, why is that an argument not to use a Mac now? How many hours a day or week do I not waste on fixing that garbage anymore? How many years can I reap the benefit of that extra time before it becomes worth while? And if I have to run a virus scanner in 5 years, well, I'll deal with that then