poliomyelitis (pōˌlēōmĪˌəlĪˈtĭs) [key], 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. Less than 1% of the people who get infected develop paralysis.
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, a preparation made from killed organisms and injected, was begun in 1955. A live-virus vaccine had earlier been developed (1948) by Hilary Koprowski, but it was never approved for use in the United States. By 1961 the Sabin vaccine, a preparation made from weakened 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 existence in other parts of the world (mainly South Asia and parts of East and West 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 later pushed back to 2005 (and even later a set deadline was abandoned), there were by 2003 less than a thousand new cases of polio worldwide, and the last last known case of type 2 poliomyelitis occurred in 1999. In 2003–4, however, the campaign was slowed when Muslim states in N Nigeria refused to use vaccines they believed would sterilize women, leading to an increase in cases there and in neighboring countries and to outbreaks of the disease in 17 countries including Yemen and Indonesia. Since then there have been other significant new outbreaks, from various sources, in some African nations and in Central Asia; the civil war in Syria resulted an outbreak there by 2013. Nonetheless, according to WHO, polio remained endemic only in Pakistan, Afghanistan, and Nigeria in 2013; early in 2012 India was declared polio free.