The Macintosh operating system enjoys a near-absence of the types of malware and spyware that affect Microsoft Windows users. This is due to both Mac OS X's UNIX roots and smaller user base. Both of these factors drive malware creators to develop for Windows. However, worms as well as potential vulnerabilities were noted in February 2006, which led some industry analysts and anti-virus companies to issue warnings that Apple's Mac OS X is not immune to viruses, as is commonly misconceived. Regardless, there has not been an outbreak of Mac malware, and Apple routinely issues security updates for its software.

Take from http://en.wikipedia.org/wiki/Macintosh#Software