Here is a nice review of the hygiene hypothesis, proposed in 1989 by David Strachan. The hygiene hypothesis is that the increases in childhood allergies and asthma in rich countries were due to decreases in “infection in early childhood, transmitted by contact with unhygenic older siblings or acquired prenatally.” It was inspired by the observation that allergies and asthma were less common in larger families.
In the original, it was infections that were the crucial thing you got from older siblings. This idea ran into trouble when actual measurements of number infections did not show the expected inverse correlation:
When a composite index of exposure was generated by combining histories of illness due to measles, mumps, rubella, varicella, and pertussis, the tendency was for a slightly higher risk of allergic disease in children with multiple infections.
Also bad for the infection idea is that vaccination for measles didn’t protect against hay fever or eczema.
It looks to my perhaps-biassed eyes that it is dirt (= harmless foreign proteins and bacteria) exposure that matters, not exposure to human infectious agents. Living on a farm helps. Plainly you get dirty living on a farm and exposed to animal viruses and bacteria — but that you get human infectious agents from pigs and cows is unlikely. (In technical terms, they aren’t vectors.) Older brothers are more protective than older sisters. Boys are dirtier than girls; it isn’t obvious they are more infectious. Dogs are more protective than cats. Again, dogs are obviously dirtier than cats but the notion that they are more infectious — few infectious agents cross the species barrier — is less obvious.
An emphasis on dirt rather than human-infectious agents is more compatible with my belief in the vast importance of ingesting bacteria-laden food.