Immune system

Why do we get sick every time we come back to Missouri? Maybe it's more about being in a new place with germs our immune systems are not used to. I don't know how to handle this. I'm all for the philosophy that uses purell and anti-bacterial sparingly because I think they create supergerms. Plain old soap and water usually seem to do the trick. Micaiah ate a lot of dirt in her first 5 months in Tanzania (not under my supervision!), and we often point to this time as the reason she doesn't often get sick-- her immune system is generally very strong! But at the same time, it doesn't seem to help against bacteria and viruses in the US... why is that?

Have any of you done research on this sort of thing? Is there a way we can guard ourselves against getting sick all the time here without living in a clean-freak bubble? Is it a matter of eating some Missouri dirt, too? Or because of our many travels, are we just doomed to a life of getting sick in every new place until we build up immunity only to start over every time we move?? Ugh.

--JLR