Youtube doesn't just take the file you upload and post THAT on their site
They encode/convert it and what-not. Then THAT is what you actually watch, which is why videos lose their quality after being uploaded. If their videos could be infected, imagine how much malware would be spread by it.Here's
a little read about how exactly they become infected. The article is a bit dated (2008), but as far as I know, the methods remain similar. I am by no means an expert, but I gather a bit of info from lurking the forums (I've been active here since 2008, this is a new account)
Edited by Sightless, 20 September 2010 - 03:57 PM.