You're right Mr Red. I don't remember any of my classmates in the early fifties having asthma. (I'd never even heard of it!)
But towards the end of my teaching career, I had a drawer absolutely full of my pupils' inhalers, each one neatly labelled with the name of the sufferer.
We weren't 'dirty' in the fifties, but didn't use anti-bacterial products or sterilise every surface/item in our homes.

Also, children played in the muck and mud outside all the time after school. No-one thought anything of it.

But as you say, there have been reputable scientific studies quite recently of Sub-Saharan African children and the level of worm infestation in more modern cities there, compared to the heavily-infested ones in the countryside.

It seems they have proved that the presence of parasites in the body stimulates defences against allergic reactions. Those who have been 'de-wormed' have more allergies.

My husband told me how as a child he often had a ghastly eruption of worms (quite large ones) pouring out of his mouth and bottom. Apparently the parasites do this from time to time.

He has a very cute abdomen, but then, I think he's cute all over!