Surely it wouldn't be too hard to write a virus that compromises both Mac based systems and *nix based systems.
You say that like you're talking about two systems, but "Mac based and *nix based" covers at least 5 different major operating system families (Solaris, AIX, *BSD, Irix, OpenServer), and that's just from what I could remember off the top of my head and not counting Linux which is a clone rather than a descendant of Unix. Compromising a Unix system is actually rather difficult compared to Windows - usually with Windows compromising one user or one service is enough to take over the system, while Unix systems tend to be more compartmentalized.