Poliomyelitis is an acute disease caused by inflammation and destruction of motor neurons after infection by a poliovirus. Sensory functions are not affected. Although frequently asymptomatic, the infection may cause fever and a number of other general symptoms, described as abortive polio or minor illness. Occasionally, however, these prodromal symptoms are followed a few days later by infection of the central nervous system (CNS) and fever, with meningitis, or paresis (weakness) or paralysis of one or more muscles. Many patients recover use of the muscle or some muscles affected in the following months, although some have permanent paralysis or paresis. When the muscles of respiration are affected, death may follow.

Other enteroviruses of the ECHO (Enteric Cytopathic Human Orphan virus) and Coxsackie groups may also cause meningitis and paresis, or temporary paralysis. In the past, cases of abortive polio and those with paralysis who later recovered were often included in statistics as polio cases. Today, only cases with paralysis or paresis after 3 months are recorded as paralytic polio.

Poliomyelitis was known by several names until the 1870s, when it became known as acute anterior poliomyelitis. Among them was Heine–Medin disease (after two early researchers, Jacob von Heine and Karl Oscar Medin) and infantile paralysis because it affected mainly young children. As more adults and older children were affected, poliomyelitis – inflammation of the gray marrow – became the name of choice and is often shortened to polio.