An acute infectious disease occurring sporadically or in epidemics and caused by a virus, usually a poliovirus but occasionally a coxsackievirus or echovirus.

It is characterized clinically by fever, sore throat, headache, and vomiting; often with stiffness of the neck and back.

In the minor illness (abortive poliomyelitis) these may be the only symptoms. The major illness, which may or may not be preceded by the minor illness, is characterized by involvement of the central nervous system, stiff neck, pleocytosis in the spinal fluid, and sometimes paralysis.

There may be subsequent atrophy of groups of muscles, ending in contraction and permanent deformity.