Polio epidemics became widespread in the late 19th and early 20th centuries as improvements in sanitation unintentionally changed patterns of exposure to the virus. In earlier periods, frequent exposure in infancy—when some protection from maternal antibodies was still present—often resulted in mild or asymptomatic infections. As hygiene improved, children were exposed later in life, when they were more vulnerable to severe disease, including paralysis. Increased urbanization and population density also facilitated the spread of the virus, leading to larger and more noticeable outbreaks that became a major public health concern.
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