I watched The Nature of Things tonight about allergies and some theories about why children are developing food allergies more frequently these days. One hypothesis holds that being overly fussy about bacteria and germs when kids are small weakens their immune systems, and that exposure to allergens at a young age can decrease allergies later.
I don't know the ins and outs of the science, but I do know what I've observed myself. Kids who grow up outdoors, on farms, with soil and animals and fresh air and germs, seem to be hearty and fresh-skinned, physically strong and a healthier weight. (They also seem to be more cheerful and fearless.) Is it the fresh air? The exposure to the bacteria in the environment? The exercise? The decreased amount of sitting-in-a-dark-basement-with-video-games? Probably a combination of all those.
With my first child, I was hyper vigilant about germs. If he dropped a pacifier or toy, I washed it before giving it back to him. I sterilized baby bottles and boiled water. With my second son, if he dropped his pacifier on the floor, I wiped it on my shirt before I gave it back. My third son kept swiping the dog's rawhide chew sticks to teethe on, until I finally bought him his own.
Son number one has several allergies and food sensitivities. Son number three is almost always healthy.
Not a scientific study, but interesting nonetheless!