Have you ever encountered infected AVI, FLV, MKV or MP3 files?
Sure have. The MP3 and AVI files from illegal downloads. I haven't seen any FLV, MKV that I know of.
Usually you download MP3, AVI or "crack" files from illegal downloads and they're infected. Then you burn / copy them to a USB device, take that USB device to work / school, plug it in and now you've infected another pc and probably infect the network servers.
File sharing P2P is the cause of most of the infections we see in the forums. Some help sites won't help you untill you've uninstalled all P2P programs.
There are good streaming services for movies, series and music and easy to use online stores nowdays so there's just no need for downloading anything illegal anymore.
Thanks for the info, LDTate!