I firmly believe that a big part of the rise in allergies in the last generation or so is because people "live too clean", and are too obsessive about avoiding any possible germ contact. Like anybody else I want to keep the salmonella out of my canned meats, and the rat excrement content of my tuna to a minimum. However, a lot of people got downright obsessive about it in the last few decades and their children were not exposed to a lot of germs and allergens growing up like earlier gens. I came up with this notion two decades ago, and I'm interested to see that some scientists have had the same idea.
When my son was small I tried to keep a clean house, as much as a single working parent can do, but I also encouraged him to get dirty outside, play with puppies and kittens, eat blackberries off the bush, and so on...without necessarily washing everything in anti-b soap first. I also sneaked now and again and sprinkled a tiny pinch of dirt in his food, deliberately.... maybe no more than three or four times in his first five years. The idea was to expose him to stuff that he was going to encounter in the environment early.
He's 16 today and has zero known allergies; healthy as a horse, rarely ill. :shrug: