Until very recently, Macs simply didn't have malware. That changed with the recent introduction of a very nasty program that puts a warning on your screen saying you have a virus. The warning is a lie but it could turn into a prediction. When you first see the it, you don't have a virus. If you believe it and download the fake antivirus software as instructed, however, you will. And that new malware will steal your personal and financial information. Big problem.
Apple has released an update that will protect against these new forms of malware, at least the ones that are in the wild. Not only should you get that pronto, make sure you have Software Update set to automatically check and install any new updates. That may not be enough but it is a start. What will help even more is if you do not believe any warning that pops up on your screen -- or comes via email. Google it to see what it really is. Prefer accounts on reliable sites such a www.pcworld.com, www.macrumors.com, www.zdnet.com or even Apple's support site (which unfortunately can be a little slow to react.)
For more information, see http://support.apple.com/kb/HT4650 and