Poliomyelitis, often called polio or infantile paralysis, is an infectious disease caused by the polio virus. In about 0.5 percent of cases there is muscle weakness resulting in an inability to move. 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.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. In up to 70 percent 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.

Polio virus is usually spread from person to person through infected fecal matter entering the mouth. It may also be spread by food or water containing human feces and less commonly from infected saliva. Those who are infected may spread the disease for up to six weeks even if no symptoms are present. The disease may be diagnosed by finding the virus in the feces or detecting antibodies against it in the blood. The disease only occurs naturally in humans.

The disease is preventable with the polio vaccine; however, multiple doses are required for it to be effective. The US Centers for Disease Control and Prevention recommends polio vaccination boosters for travelers and those who live in countries where the disease is occurring. Once infected there is no specific treatment. In 2016, there were 37 cases of wild polio and 5 cases of vaccine-derived polio. This is down from 350,000 wild cases in 1988. In 2014 the disease was only spreading between people in Afghanistan, Nigeria, and Pakistan. In 2015 Nigeria had stopped the spread of wild polio virus but it reoccurred in 2016.

Poliomyelitis has existed for thousands of years, with depictions of the disease in ancient art. The disease was first recognized as a distinct condition by the English physician Michael Underwood in 1789 and the virus that causes it was first identified in 1908 by the Austrian immunologist Karl Landsteiner. Major outbreaks started to occur in the late 19th century in Europe and the United States. In the 20th century it became one of the most worrying childhood diseases in these areas. The first polio vaccine was developed in the 1950 by Jonas Salk. It was hoped that vaccination efforts and early detection of cases would result in global eradication of the disease by 2018.