Historically, Mac users haven’t had to worry about malware as much as their Windows-using cousins. Although malware targeting Apple devices actually predates viruses written for PCs, and there have been some families of malware that have presented a significant threat for both operating systems (for instance, the Word macro viruses that hit computers hard from 1995 onwards), it is generally the case that you’re simply a lot less likely to encounter malware on your Mac than you are on your Windows PC.

Source: Tripwire – The State of Security