I kept an article from our local newspaper, May 31, 2003, entitled "Want healthy children? Let 'em eat a little dirt." It's hypothesis is that "early childhood exposure to dirty conditions, including bacteria and other disease-causing germs is important for normal development of the immune system."
Speaking for myself and my family, I think there's some validity to this theory. I remember one day when my daughter (then 2) picked up and ate a rolly-polly bug while playing in the front yard! That happened only once, and of course we discouraged her from repeating the "game." Both of my children are now college freshmen and have seldom been sick. (She has more problems with monthly cramps than with any illness.)
Conversely, those peers of my children who received drugs and/or doctor-visits for every minor cold, cut or problem seem to have developed more allergies and/or tendancy toward illness as adults.
I believe there's something to be said for this theory that exposure to "dirty conditions, including bacteria" during childhood does go a long way toward building up the immune system of that person in adulthood!