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. There are two forms of polio: Minor poliomyelitis (also called abortive poliomyelitis) occurs primarily in young children, and is the more common of the two forms. The illness is mild, and the brain and spinal cord are not affected. Symptoms appear three to five days after exposure to the virus and include slight fever, headache, sore throat, vomiting, lack of appetite, and a general feeling of illness and discomfort. Major poliomyelitis is a more severe illness that develops approximately 7 to 14 days after exposure to the virus. Symptoms include fever, severe headache, stiff neck and back, and deep muscle pain. Some people experience temporary abnormalities of skin sensation. Muscle spasms and a tendency to retain urine are common. Muscle weakness and paralysis may develop rapidly or gradually during the time fevers occur, but paralysis does not continue to get worse afterwards. The disease most commonly affects the strength of muscles of the legs. It can also affect strength in the muscles of the arms and abdomen. When polio affects strength in the muscles of the neck and throat, it causes difficulty speaking and swallowing. The most life-threatening form of polio causes weakness of the muscles in the chest that are needed for breathing. The virus also can sometimes affect the parts of the brain that control breathing. When a polio victim develops breathing trouble, they need machines to help breathe for them.  
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