The theory is that constant low level exposure to the polio microbe allowed a body to develop immunity. When indoor plumbing came on the scene then outbreaks of polio rose dramatically when the microbe was present in the environment due to a exposure of a large enough dose to cause an infection. It can be searched but the subject gets lost in the vaccine flame wars on the interwebs.
Odd that I didn’t come across.that while researching Polio and “other” factors.
The FDR story is compelling and has a lot of.supporting data.
More research...