Zeb, there
are virii and trojans and rootkits for macintosh systems, it's just they are typically not distributed widely or through emails, different OSes are targeted different ways, macs, a good option would be pirated software, for windows, it's email, ims, websites, software, music, just about anything

doesn't mean there aren't any, just they are less targeted
basic rule of thumb for a mac is....if it's a .exe file, you're safe