Poliomyelitis (polio) is a highly infectious disease caused by a virus that invades the nervous system.

Up to 95% of persons infected with polio will have no symptoms. About four to eight percent of infected persons have minor symptoms such as fever, fatigue, nausea, headache, flu-like symptoms, stiffness in the neck and back, and pain in the limbs which often resolves completely. Fewer than one percent of polio cases result in permanent paralysis of the limbs (usually the legs). Of those paralyzed, 5-10% die when the paralysis strikes the respiratory muscles.