I don't think there are operating systems that are NOT vulnerable to viruses or don't have viruses.
Mac user population is significantly less than windows user population, therefore majority of viruses go to windows not mac or something else.
Agreed that there aren't any OS's that aren't vulnerable to viruses or have them.
However, unix (thereby Linux and Mac OS X) are much more secure than Windows, regardless of user population.
That being said, if Linux or Mac OS X held the majority of user base like Windows does, there would be more incentive for hackers to find security holes and we'd definitely see more viruses on those OS's.