Re: mac AV
There has actually been a couple of widespread file infectors for the NT line of Windows (NT/2000/XP/Vista/7/etc...) but I haven't seen them in any list of most widespread malware in quite a few years. Also they were more of hybrids - file infector, worm spreading via USB sticks and network, etc.
I suspect they died out post XP where you no longer run everything as Administrator by default.
And remember, all mass-distributed malware nowadays has a commercial purpose. File infectors tend to be far too noisy and have too many side effects to monetize well.
Biting the hand that feeds IT
