小儿麻痹症（Poliomyelitis、Polio；中文译自日语小児麻痹）又译急性灰白髓炎、脊髓灰质炎。是由脊髓灰质炎病毒引起，可感染人类之病症。大约有90%到95%的感染并没有任何症状。剩下5%到10%有发烧、头痛、呕吐、腹泻、颈部僵硬以及四肢疼痛等轻微症状。这些有症状的患者往往在一到两周内就会完全复原。只有约0.5%的患者会发生肌力变弱而导致行动困难。疾病的进程可能只是数小时，也可能需要数天。肌力变弱的状况往往发生在下肢，很少发生在头部、颈部、横膈膜的肌肉。大多数的人可以自行复原，但少数的人会造成终生残疾。感染此病毒造成肌力变弱的患者中，有2%到5%的幼年患者与15%到30%成年患者会导致死亡。从后小儿麻痹感染症候群中恢复，需要很长的时间，会很长一段时间都有如同感染时期一样的肌力不足。

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 that which the person had during the initial infection.