It's the nature of children
to touch anything that their parents find disgusting. Just try to keep your baby and dog from sharing a meal and salivary secretions!
Recent research suggests that exposing infants to germs may offer them greater protection from illnesses, allergies and asthma later on in life. A current article in the NY Times serves as a user-friendly introduction to this issue. Your Environment Is Cleaner. Your Immune System Has Never Been So Unprepared. NY Times, March 12, 2019 by Matt Richtel
(Excerpts) A 1989 BMJ paper, based on a study of over 17,000 children born in 1958 hypothesized that “allergic diseases were prevented by infection in early childhood, transmitted by unhygienic contact with older siblings, or acquired prenatally from a mother infected by contact with her older children.*
Thanks to all the powerful learning we’ve done as a species, we have minimized the regular interaction not just with parasites but even with friendly bacteria and parasites that helped to teach and hone the immune system — that “trained” it. It doesn’t encounter as many bugs when we are babies. This is not just because our homes are cleaner, but also because our families are smaller (fewer older children are bringing home the germs), our foods and water cleaner, our milk sterilized.
Food and respiratory allergies rose in tandem with income level. More money, which typically correlates with higher education, has meant more risk of allergy. This may reflect differences in who reports such allergies, but it also springs from differences in environment.
* Hayfever, Hygiene and Household Size, David Strachan, BMJ 1989