Poliomyelitis, often called polio or infantile paralysis, is an infectious disease caused by the poliovirus. In about 0.5% of cases there is muscle weakness resulting in an inability to move. This can occur over a few hours to few days. The weakness most often involves the legs but may less commonly involve the muscles of the head, neck and diaphragm. Many but not all people fully recover. In those with muscle weakness about 2% to 5% of children and 15% to 30% of adults die. Another 25% of people have minor symptoms such as fever and a sore throat and up to 5% have headache, neck stiffness and pains in the arms and legs. These people are usually back to normal within one or two weeks. In up to 70% of infections there are no symptoms. Years after recovery post-polio syndrome may occur, with a slow development of muscle weakness similar to what the person had during the initial infection.