As of today, there have been exactly two documented, successful pieces of malicious software — viruses, trojan horses, worms — that affected users of the Mac OS X operating system, since it was released in 2001. And these two failed to spread much, affecting probably a few dozen people, and doing no harm. I expect there to be a small number of additional Mac viruses this year.
By contrast, there are over 100,000 reported viruses for Windows, some of which have affected millions of people and have done significant economic damage. As for spyware, I know of no documented cases on Mac OS X, while there are certainly thousands on Windows. These Windows viruses and spyware can’t run on the Mac operating system, even on Macs powered by the same Intel processors used by Windows PCs.