It was DDT. DDT is a neurotoxin designed to kill insects by destroying their ability to send signals and causing tremors and convulsions. These happen to be the same symptoms as Polio, and we used to spray DDT on kids and all over crops. Doctors couldn't distinguish the symptoms. Basically, DDT was causing neurological damage that was getting mislabeled as Polio.

They significantly tapered down DDT a year or two before the polio vaccine, which is why you can see the rates of cases drop like a stone before the vaccine saw a wide scale rollout.