There are OS X viruses out there. Linux would be harder because of an increased fragmentation and systems being more up-to-date generally (especially since linux is commonly used on servers, and thus held up to a different standard of security in first place)

On windows or mac, getting a virus to run is generally just a matter of packaging it with some downloadable software. In linux, pretty much everything is available through package managers. You can't just throw a virus into a torrent for photoshop or whatever.

That being said, I'm by no means an expert.