What Is It?
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.