“All viruses and germs ebb and wane.”
True, but I have no idea to which you’re referencing re indoor plumbing. No correlation whatsoever. Reference?
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.