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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

Polio is a highly contagious infection caused by the poliovirus. In a small percentage of infected people, the virus attacks nerve cells in the brain and spinal cord, particularly the nerve cells in the spinal cord that control muscles involved in voluntary movement such as walking. Destruction of these neurons causes permanent paralysis in 1 in 200 cases. Polio is also called poliomyelitis.

The infection spreads through direct contact with virus particles that are shed from the throat or in feces. The disease has been virtually wiped out in the Western hemisphere since the introduction of the inactivated vaccine in 1955 and the oral, live vaccine in 1961. Vaccination campaigns have succeeded in reducing the number of countries where polio is endemic (where it occurs locally). In 1988, more than 120 countries contained endemic poliovirus; by 1998, only 50 countries contained endemic polio; by 2002, only 6 countries still had locally circulating infection. There were 1,265 confirmed cases of polio worldwide in 2004. Today, 90% of all endemic polio is contained within India, Nigeria and Pakistan. In developing countries, some people remain unvaccinated. Poor sanitation and poor hygiene promote the spread of the virus. People traveling to these areas of the world must have up-to-date immunizations. The World Health Organization is attempting to eradicate polio worldwide, as it did with smallpox.

Though rare, polio has been caused when people are immunized with the live polio vaccine. Countries that have wiped out polio usually use the inactivated polio vaccine, which never causes polio.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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