As insane as this sounds it is perfectly logical. If you are exposed to harmful and beneficial bacteria from youth, your body develops immunity to the bad ones and incorporates the good ones into your intestinal flora.
Makes good sense to me.
Western Europe, North America and Japan are the most hygiene societies on earth. We also have the highest incidences of immune reactions on earth. This is not a coincidence.
What I have also found odd is the polio epidemic in the sixties. In Texas it was wide spread as in the rest of the United States. However just across the border in Mexico it was not and in the sixties there was much cross border of people as today. Mexicans seemed somewhat immune though some did catch the disease but in far fewer numbers than Anglos living side by side.
Little kids will eat dirt. There must be a reason. Unfortunately there is not much dirt in big cities.