IIRC, if it's respiratory, it's usually through touch contact - door handles, the backs of chairs, tables, badges, money, etc. - any surface that you touch and someone else touches. Or it can be passed through the air by little droplets of water, which brings in the air conditioning (Legionnaire's disease was passed that way at a hotel when the first outbreak happened). I got a cold after an Animefest a few years ago when I was walking by a group of people and one of them turned and sneezed right on me. (THIS IS WHY YOU COVER YOUR NOSE AND MOUTH WHEN YOU DO THAT AARG)

If it's gut-related, it could be any of the above, plus someone who's infected and preparing or touching food in any way.

I'm sure someone who knows more about epidemiology than I do will come by and correct me, though. :D

ETA: (After I just went upstairs to get chips - ELEVATOR BUTTONS)