At a con? The elevator buttons are constantly in use, especially the ones on the outside that call the elevator.

I don't know why you don't get sick during the first week of school, but it's possible that your campus has a smaller percentage of people from other areas, so you're slightly more resistant to the bugs. Or you are in less confined areas, so the moisture that harbors the bugs dries up before you touch the surface or breathe it in (cons pack 'em in tight). You may also be getting more sleep and better nutrition during the first week of school than at a con.