I don't know ... I've given this a lot of thought too. I'm probably older than a lot of you, and I honestly think that in many ways, we were healthier back then than a lot of today's kids are. When I was a kid, we never heard of "peanut allergies". A few kids had "allergies" but more the seasonal type - my dad called it "hay fever". I had it too and it meant you took an extra big hankie to school to blow your nose in! There were a very few diabetic kids, a very few with minor food allergies, but I don't remember anyone having anything really serious - certainly no kids dying from "peanut allergies". And I'm sure there was such a thing as autism, but it was very, very rare.
But we lived a very different lifestyle back then - not discounting the tremendous advances in modern medicine - but we were a much more active and less "clean" and sanitized generation and I wonder if we didn't build up more immunities to certain things. We only went to the doctor if we were really sick and our parents were worried that we weren't getting over it on our own. We got immunizations, but nowhere near what they get today - and now kids are getting even more than my own kids did not that many years ago. There was no such thing as "hand sanitizer" and nobody worried about removing all the bacteria from the air we breathed - and most of our parents smoked like chimneys! Not that that was good, but that's the way it was! We spent all day outside climbing trees, riding bikes and rolling in the grass. We dug in the dirt and held worms in our hands - and sometimes a Twinkie in the other hand. You washed your hands before meals unless you could fake it and a lot of times we did! There was no "hypo-allergenic" anything and we got by and adapted. And to this day, I still have the worlds greatest immune system! I hardly ever get sick and I think I've taken antibiotics maybe once in the last twenty years! There has to be something to it.
I honestly don't know what the answer is. But it does seem like we were a much heartier breed back then and that each generation that comes along becomes a little less so.