Hmm interesting. Do you have a source for this?
I am guessing the higher rank on linux is because of the server market share. But even then the way I see it is on windows, attacks are targeted at the core windows operating system components. Whereas on linux, attacks seem to not be targeted at linux in general, but programs and services that run on linux like openssh and apache. Thats just how it seems atleast. I certainly don't think there are any viruses for linux (or mac)