I agree there is not to much to worry with a virus attacking a mac. I practily dip mine in a virus on windows to see what happens.
Its just that a Virus on Windows computers are developed in a different language then Mac. Mac's are Objective-C programing while programing for Windows is C Sharp. So if you introduce a virus from Windows to Mac the language will not understand it.
However that is just my opinion and not a fact I think