[michaelt]

While I was at university, every lecture hall data projector had a PC connected to it. Often people would bring their presentations from home on USB memory sticks.

Nowerdays there are viruses that spread by USB memory stick - and lie dormant on the computer infecting every USB memory stick that gets plugged in. Needless to say, lecture hall computers quickly became infected - even without any malicious intent on the part of the physically present user.