With computer viruses, the situation is a little different. Put an infected computer near a "healthy" computer, and nothing will happen.
Today I was at the photo-printing shop with my disk-on-key. Nothing special, as we all print photos this way nowadays. When I got back home my DoK contained 7 new unrecognized .exe files, and an "autorun.inf" file. Hmmm... I don't remember putting those there. A quick virus scan, and viola, 7 torjans/droppers/viruses found their way to the DoK in the few minutes it was mounted at the computer at the photo-printing shop. Good thing I'm safe, but I guess many other get infected after printing their photos in this store. Ouch.