See: Polio https://en.wikipedia.org/wiki/History_of_polio https://en.wikipedia.org/wiki/History_of_polio > [...] Better hygiene meant that infants and young c

See: Polio https://en.wikipedia.org/wiki/History_of_polio https://en.wikipedia.org/wiki/History_of_polio > [...] Better hygiene meant that infants and young children had fewer opportunities to encounter and develop immunity to polio. Exposure to poliovirus was therefore delayed until late childhood or adult life, when it was more likely to take the paralytic form.[22]