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Why did polio epidemics become more common after sanitation improved in Europe and North America?

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Polio vaccine — polio A health care worker giving a polio vaccine to a child in Katsina state, Nigeria, 2014.

It is a historical irony that polio epidemics became more frequent in industrialized nations as hygiene and sanitation standards improved during the 19th and 20th centuries.1

In earlier, less sanitary environments, infants were typically exposed to the virus at a very young age. During this period, they benefited from residual maternal antibodies, which often resulted in mild or asymptomatic infections that provided natural immunity.12

As sanitation improved, the frequency of early-childhood exposure decreased. Consequently, children and adults were exposed to the virus later in life, when they lacked maternal protection and were significantly more vulnerable to severe outcomes, including paralysis. This shift created a large population of susceptible individuals, allowing the virus to spread rapidly through urbanized areas and trigger the widespread epidemics that characterized the mid-20th century.12