The Mac is immune to viruses.
Viruses, spyware, and trojans that affect the Macintosh operating system are few and far between, but they do exist.
One example – DNSChanger. This is a trojan horse (program that masquerades as another – named after Hector’s ruse in Homer’s Iliad) for Mac OS X. It is also known as OSX.RSPlug.A.
DNSChanger, once installed, hijacks sites you visit in your browser. You might think you’re on amazon.com, but you’re actually on another server that just wants to serve more ads to you, or worse, convince you to order something, entering your credit card info in a fake order page.
How do you “catch” DNSChanger? It usually comes from questionable web sites which convince you to install a piece of software they call a video codec, ostensibly to watch their video.
That said, I still don’t normally recommend antivirus software for the Mac, unless your goal is to protect Windows users from other Windows users – i.e. from malware that might be forwarded to you, and then to someone else via email.
The best thing a Mac owner can do to avoid viruses and spyware and such is to use common sense and surf carefully.
If a web site recommends a special piece of software to see its contents (such as a video codec or player), think twice before installing it. If you need QuickTime, go ahead and install it, but get it directly from Apple.
Same for Flash – get it directly from Adobe.
Products or Codecs you’ve never heard of? Pass, or at least do some research first. If a site recommends the xxxyyzz product, google “xxxyyzz” and see what other reputable sites have to say.