yes. in essence, different os have different loopholes. windows takes up wad. 90% of the market share of os? if u wanna make viruses u go for the one tat can hurt more wad.
but.
mac os does have its own weaknesses. i havent gotten myself a mac. so far only been using sch one only. but as far as i can tell, it has loads of loopholes tat are easy to break through. one example will be jailbreaking ur apple iphone os. if u can do tat (which is a form of hacking in itself), i see no reason why u can't hack pass the mac osx.
possibly the only OS i could really claim as virus free is linux ubuntu. its opensource, aka ppl candrop by and always patch up. think abt it. compared to windows or mac which have only a few ppl looking and patching loopholes, linux users are kind ppl, a lot of them actually, who share what they know and make the os better. and its free, by the tech ppl for the tech ppl.