Boys raise money for the fight against Polio. United States, circa early 1950's. Polio is an acute infectious viral disease that can cause muscle paralysis and death. It is spread by close personal contact, usually from saliva or waste. Polio reached epidemic proportions only in the 20th century, and only in developed countries. The disease, endemic in the human population for centuries, had in the past silently infected most infants and had spread from child to child to create a natural immunity through early exposure. Now, due to modern sanitation, polio struck older children and adults who had never been exposed to and had no protection from the virus. In 1952 alone, more than 57,600 Americans contracted polio. In 1955, Jonas Salk's polio vaccine put an end to the polio epidemic.