[Torin]

Tollund in a CD or in a flash usb memory there can be a file called
autorun.inf
being this a text file stating wich applications will run when the media is inserted.
it can be anything. An exe file for example.
If you copy in your flash usb 2 files:

autorun.inf
virus.exe

and set the 2 files hidden. the next fool that puts that flash into the usb port will be infected no matter what (probably the AV will stop infection if the AV know the existence of that virus).

The worst part that you thought the flash mem was empty.
"Autorun on" and "dont see files hidden" are default options of windows.
you should put "autorun off" and "see hidden files" if you want to see what can infect you.

And instead of a virus you can insert a call to shutdown computer mind you.