In children, paralysis due to polio occurs in one in 1000 cases, while in adults, paralysis occurs in one in 75 cases. By 1950, the peak age incidence of paralytic poliomyelitis in the United States had shifted from infants to children aged 5 to 9 years; about one-third of the cases were reported in persons over 15 years of age. Accordingly, the rate of paralysis and death due to polio infection also increased during this time. In the United States, the 1952 polio epidemic was the worst outbreak in the nation's history, and is credited with heightening parents’ fears of the disease and focusing public awareness on the need for a vaccine. Of the 57,628 cases reported that year 3,145 died and 21,269 were left with mild to disabling paralysis.