pōˌlēōmīˌəlīˈtĭs, polio, or infantile paralysis, acute viral infection, mainly of children but also affecting older persons. There are three immunologic types of poliomyelitis virus; exposure to one type produces immunity only to that type, so infection with the other types is still possible. Spread of the infection is primarily through contact with an infected person. Most people who contract polio either exhibit no symptoms or experience only minor illness; however, such individuals can harbor the virus and spread it to others.

The virus enters the body by way of the mouth, invades the bloodstream, and may be carried to the central nervous system, where it causes lesions of the gray matter of the spinal cord and brain. The illness begins with fever, headache, stiff neck and back, and muscle pain and tenderness. If there is involvement of the central nervous system, paralysis ensues. Of those patients who develop paralytic poliomyelitis, about 25% sustain severe permanent disability, another 25% have mild disabilities, and 50% recover with no residual paralysis. The disease is usually fatal if the nerve cells in the brain are attacked (bulbar poliomyelitis), causing paralysis of essential muscles, such as those controlling swallowing, heartbeat, and respiration. There is no specific drug for treatment. For reasons not clearly understood, some people who have had severe polio experience postpolio syndrome, a condition in which new weakness and pain occurs years later in previously affected muscles.

The incidence of poliomyelitis declined radically in the United States when a mass immunization program with the Salk vaccine was begun in 1955. By 1961 the Sabin vaccine, a preparation made from living organisms and taken orally, was released for use. Since then the disease has been virtually eliminated in the Americas, Europe, and Australasia, but vaccination programs continue because of polio's continued existence in other parts of the world (mainly South Asia and parts of Africa) and the ease of travel. In 1988 the World Health Organization began a global vaccination campaign to eradicate the disease—which continued to paralyze hundreds of thousands of children each year—by 2000. Although the date of eradication was pushed back to 2005, by 2001 there only 600 new cases of polio worldwide.

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved.

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