I'd also be curious about children who grow up in rural, suburban and urban areas.

I live in New York, I often forget to wash my hands when I come home from riding the subway and start hugging my kid one of whose favorite games is "chew on dad". I'm trying to balance out the fear of her catching some exotic imported disease with the hope that her immune system is getting used to handling a variety of invaders.

You end up with highly contagious pathogens that you'd never find in your home (Hand, Foot & Mouth, for example) and higher incidence of things like conjunctivitis, but since everyone in licensed group care facilities is vaccinated against the real baddies, that stuff is no more frequent. What parents bank on is that the persistent coughs & colds that are so common will eventually lead to stronger immune systems. As a parent of 2 elementary school kids who both did group daycare/preschool, I have 0 useful data to share. My kids got sick regularly in preschool, don't get sick as often in elementary school, but when one gets sick (with a viral infection) generally everyone else in the family still gets sick, too.

Probably a lot more hand sanitizer and other precautions that aren't in your house too.

The type of germs that kids share with each other are different to what you can pick up from the dirt. I suspect (though I would love to find evidence) that upper respiratory infections are far more common today than they were in the past. We now have dense cities, global travel, compulsory schooling, and a large global population for colds and flus to diversify in.

On the other hand we have less exposure to the bacteria and parasites that live in the dirt and on animals. They trigger (and modulate) a different part of the immune system associated with allergies.