Before the introduction of the polio vaccine, polio was a common childhood disease. It was most know for infantile paralysis and mostly affected young children although it can affect young adults and older people. In developed countries where standards of hygeine and sanitation the incidence of infection overall has decreased. However, it has prevented most people from acquiring immunity as a child when it is less likely to cause paralysis. In the most severe cases this can affect the muscles involved in breathing and other important functions which can lead to death.
Most infected children are asymptomatic.
Severity increases with age. Nonlethal infections recover fully with no permanent liver damage.