For a year or so now I've wondered about the whole "Modern Life causes food allergies" claim that is being tossed around. I've heard it's because we eat too much processed food, because we don't play in the mud enough, because we go to doctors for all our problems so our immune systems are too weak. While the proponents of these ideas make strong arguments sometimes, the arguments don't seem to answer all the questions or clarify all the data exactly right.
Today it dawned on me:
Our grandparents didn't know anyone with food allergies because those people died the first time they had a bad reaction to food, which is usually in childhood. Modern medicine allows us to save the life of people with food allergies, so you meet them now. And they live to bear children, which would allow them to produce more people who potentially have food allergies.
People also didn't go to the doctor as often back then, for sure. And they died of that, too.
If you eliminate all the people with food allergies from the population by killing them off (or allowing them to die naturally, from their allergies), then of course you had fewer food allergies showing up in the population.
Also, people's diets were far more restricted--there was less choice by far. That means people were statistically less likely to be exposed to all the food allergens, giving them less chance to discover they had a food allergy.
So you either died from the food allergy right away or never found out you had one. There may have been just as many food allergy families back then--they just didn't know it or didn't survive to have children with more food allergies.