Almost all viruses and other bad things that can attack computers are designed for PC's - not Macs. I was talking to my local Mac repair shop about this and he said he literally NEVER has to do repair work because of viruses, etc., and that he doesn't even sell the virus protection software in his store.
This isn't necessarily because Mac OS is any more secure, just that the Windows OS is far more prevalent and building viruses to that take advantage of that software has a far more wide spread effect. Mac OS isn't immune to viruses.