There's speculation that many allergies are due to underworked immune systems. They're correlated the increase in allergies to the increase in wealth and cleanliness in a number of countries around the world.
I believe it. We washed our hands before we ate, and my mother used the same cutting board for meats, vegetables and bread.
We all got colds and once in a while, we’d get the flu, but not too often. The rest of the time, it was the “childhood diseases” (including pertussis when I was two) and we all survived.