There is a fairly widely accepted notion that the great polio epidemic was the direct result of the spread of indoor plumbing. There were/are I believe(?) 4 variants of the "bug," 3 of which were mild but kept a general resistance prevalent, which prevented widespread infection by the one serious variant. "Improved" sanitation wiped out the "inoculant" varieties, leaving everyone susceptible to the paralytic one. Once the nasty one reached "critical mass" it spread virtually everywhere.

Less accepted, but still open for further research, is that the apparent surge in "asthmatic conditions" in the population may be the result of "overly fastidious" protection of small children from "dirty things" that could have innoculated them while their immmune systems are developing. Apparently valid research has indicated that asthma and similar "allergic problems" are much less common when there were pets in the household while the children were small; but it seems to have little effect beyond about age 2.

John