Poliomyelitis, commonly shortened to polio, is an infectious disease caused by the poliovirus. In about 0.5 percent of cases, it moves from the gut to affect the central nervous system and there is muscle weakness resulting in a flaccid paralysis. This can occur over a few hours to a few days. The weakness most often involves the legs, but may less commonly involve the muscles of the head, neck and diaphragm. Many people fully recover. In those with muscle weakness, about 2 to 5 percent of children and 15 to 30 percent of adults die. Up to 70 percent of those infected have no symptoms. Another 25 percent of people have minor symptoms such as fever and a sore throat, and up to 5 percent have headache, neck stiffness and pains in the arms and legs. These people are usually back to normal within one or two weeks. Years after recovery, post-polio syndrome may occur, with a slow development of muscle weakness similar to that which the person had during the initial infection.