The immune system is a learning machine, and the more we've been exposed to the more efficient it becomes. I take that to heart and I seldom avoid germs or sick people, unless I feel weakened (no sleep, stressed, high blood sugar) or they've got something particularly nasty. I almost never get sick and I never use sick days. I haven't been to a non-endo doctor in at least a decade. Everyone's constitution is different and so above all else it's important to listen to your own body in this regard.
Oh, and I've noticed that If my blood sugar gets over 400 and I haven't slept then it's almost guaranteed I'll get sick regardless of obvious germs, but fortunately that's a very rare occurrence.