The only reasons Mac's don't get viruses is because hackers don't care about them enough to make viruses for them.  Simply put, the more software that a computer has and the more that software uses the internet, the more vulnerable it is.  PC's often come bundled with a bunch of stuff you don't need that is later found out to make your computer vulnerable.  It's no different for Macs, and even a release of Ubuntu Linux was found to be extremely vulnerable recently.

Since most viruses use holes in OS software to get into your computer, a virus for say, Windows Office, isn't going to effect a Mac.

if Mac OS were to suddenly gain a significant marketshare and be worth hackers attacking, you'd see that it is probably less secure than a PC because it hasn't been beat on by hackers for years and years.