Polio is a highly contagious infection caused by the poliovirus. Most people infected with the virus develop no symptoms from it. However, 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. Permanent paralysis occurs in one out of every 200 cases of polio. 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 polio vaccine (the "Salk vaccine") in 1955 and the oral, live vaccine (the "Sabin 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 2012, only 3 countries contained endemic polio.
However, in 2013 new cases of polio developed in countries in the developing world where the virus had appeared to have been eradicated, including countries in the Horn of Africa. The virus also reappeared in developed countries in regions of the world swept by violence and social disruption, including Syria and Israel.
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.
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.