In the late 1800s in the United States, babies started developing scurvy; there was a veritable plague. It turned out that the vast majority of victims were being fed milk that had been heat treated (as suggested by Pasteur) to control bacterial disease. Pasteurization was effective against bacteria, but it destroyed the Vitamin C.
From a history of nutrition. Now children are probably getting all sorts of immune disorders, such as hay fever, for the same core reason: fear of bacteria.
Depending how long “witholding” bacteria from the children was needed to cause an immune disorder a convenience sample could be a study of the rates of immune disorders in the population of children who had some illness as an infant (e.g. I had open-heart surgery) that are now well as adults. In my case I was kept away from bacterias, viruses, illnesses etc for the first ~2 years as I had a diminished ability to fight off diseases due to the heart defect.
If the hypothesis is correct these children would likely have higher rates of immune disorders (unrelated to the specific childhood illness) than the general population.
I have quite a few airborne allergies (trees/grasses etc) which would be anecdotal supporting evidence, but so does my sister who did not have a significant childhood illness so maybe it is purely environmental/parenting…
Scott, the sad fact is that almost all children, probably including your sister, are kept away from harmless bacteria, which are in fermented food above all. I mean, the amount of harmless bacteria that would enter your body by eating fermented food daily would dwarf all other sources. Your deprivation may have been only slightly more than your sister’s. Unless she regularly ate fermented food growing up — but I’m guessing she didn’t.