Polio was one of the most feared diseases of the 20th century. In 99% of cases, it causes no symptoms or very mild symptoms and may go effectively unnoticed, making it easy for people to be unwitting carriers and spreaders of the virus and leading to an explosion of polio epidemics in the early 20th century as transport improved, population density in cities increased and it became easier for a virus like polio to reach a critical mass of people. Meanwhile, in that last 1% of sufferers polio infects the spinal cord, leading to paralysis or death. Even worse, young children tends to be particularly susceptible, leading to polio becoming known in some areas as infantile paralysis disease. There is still no known cure for polio.
In 1952 in the United States alone, polio caused 3,145 recorded deaths and 21,269 recorded cases of mild to disabling paralysis.