I can't find a reference right now, but delayed exposure is thought to be one reason why polio turned into a severe disease. Polio has been endemic in humans fo

I can't find a reference right now, but delayed exposure is thought to be one reason why polio turned into a severe disease. Polio has been endemic in humans for generations, but it started causing severe disease in the late 1800's/early 1900's. Previously, infants were exposed to polio, but increasing hygiene meant that exposure would be delayed. Again, I can't find a reference for this now, so I'm probably getting details wrong.