Here is the CDC saying it:
> Before the 18th century, polioviruses probably circulated widely. Initial infections with at least one type probably occurred in early infancy, when transplacentally acquired maternal antibodies were high and protected infants from infection-causing paralysis.
> In the immediate prevaccine era, during the first half of the 20th century, improved sanitation resulted in less frequent exposure and increased the age of primary infection, resulting in large epidemics with high numbers of deaths. The incidence dramatically decreased after the introduction of inactivated polio vaccine (IPV) in 1955 and continued to decline following oral polio vaccine (OPV) introduction in 1961. From the more than 21,000 paralytic cases reported in 1952, only 2,525 cases were reported in 1960 and 61 cases in 1965.
> Poliovirus Secular Trends in the United Status:
> * Before the 18th century, polioviruses probably circulated widely
> * In immediate prevaccine era, improved sanitation resulted in less frequent exposure and increased age of primary infection, resulting in large epidemics with high death count
> * Incidence dramatically decreased following inactivated polio vaccine (IPV) introduction in 1955