Why did the polio epidemics seem to hit hardest those who were better off hygienically?

Polio epidemics often hit harder in communities with better sanitation and hygiene because these areas experienced lower levels of exposure to the poliovirus during early childhood. In cleaner environments, infants were less likely to encounter the virus, which meant they did not build natural immunity. When outbreaks occurred, older children and adults—who had not been exposed earlier—were more vulnerable to severe cases of the disease. Thus, affluence and hygiene inadvertently contributed to the severity of polio outbreaks in these populations.