I saw something once, and I don't remember where (it was years ago) and so I don't know how much truth there is to it. However, apparently some science joint dos a big scale allergy test on x amount of kids from the US and x amount of kids from some tribal villages where kids were drinking river water and eating food from tree or wild beasts they killed. Apparently the difference in results was not even close. The tribal children had almost zero allergies where as the US results were really high.
Again, not sure where I read it or how truthful, but I found it interesting and I can see some merit in it.