Post-polio syndrome (PPS, or post-poliomyelitis syndrome or post-polio sequelae) is a condition that affects approximately 25–50% of people who have previously survived an acute attack of poliomyelitis —a viral infection of the nervous system—after the initial infection. Typically the symptoms appear 15–30 years after recovery from the original paralytic attack, at an age of 35 to 60. Symptoms include acute or increased muscular weakness, pain in the muscles, and fatigue. The same symptoms may also occur years after a nonparalytic polio (NPP) infection.