See: 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]

Nothing strengthens an immune system like a weekly furry party or attending "tough mudders" events.

Thank you! I wish I could upvote this twice, fellow redditor!