There has been some reports on the radio lately that say a recent study found the best way to keep your child from getting peanut allergies was to give it a peanut product, like peanut butter, in the first year of it's life.
To me it seems that avoiding bacterial infections is similar. The best way to keep your kids healthy in adult life is to give them some time outside to coexist with the bacteria so their bodies can get used to it.
If you see what I mean, I don't explain myself very well.
I'm not suggesting you let them swim in the septic tank or anything like that.