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( Polio)
Poliomyelitis, often called polio or infantile paralysis, is an acute viral infectious disease spread from person to person, primarily via the fecal-oral route.[1] The term derives from the Greek polio (p?????), meaning "grey", myelon (µ?e???), referring to the "spinal cord", and -itis, which denotes inflammation.[2] Although around 90% of polio infections cause no symptoms at all, affected individuals can exhibit a range of symptoms if the virus enters the blood stream.[3] In fewer than 1% of cases the virus enters the central nervous system, preferentially infecting and destroying motor neurons, leading to muscle weakness and acute flaccid paralysis. Different types of paralysis may occur, depending on the nerves involved. Spinal polio is the most common form, characterized by asymmetric paralysis that most often involves the legs. Bulbar polio leads to weakness of muscles innervated by cranial nerves. Bulbospinal polio is a combination of bulbar and spinal paralysis.[4] Poliomyelitis was first recognized as a distinct condition by Jakob Heine in 1840.[5] Its causative agent, poliovirus, was identified in 1908 by Karl Landsteiner.[5] Although major polio epidemics were unknown before the late 19th century, polio was one of the most dreaded childhood diseases of the 20th century. Polio epidemics have crippled thousands of people, mostly young children; the disease has caused paralysis and death for much of human history. Polio had existed for thousands of years quietly as an endemic pathogen until the 1880s, when major epidemics began to occur in Europe; soon after, widespread epidemics appeared in the United States.[6] By 1910, much of the world experienced a dramatic increase in polio cases and frequent epidemics became regular events, primarily in cities during the summer months. These epidemics—which left thousands of children and adults paralyzed—provided the impetus for a "Great Race" towards the development of a vaccine. The polio vaccines developed by Jonas Salk in 1952 and Albert Sabin in 1962 are credited with reducing the global number of polio cases per year from many hundreds of thousands to around a thousand.[7] Enhanced vaccination efforts led by the World Health Organization, UNICEF and Rotary International could result in global eradication of the disease.[8] Poliomyelitis is caused by infection with a member of the genus Enterovirus known as poliovirus (PV). This group of RNA viruses prefers to inhabit the gastrointestinal tract.[1] PV infects and causes disease in humans alone.[3] Its structure is very simple, composed of a single (+) sense RNA genome enclosed in a protein shell called a capsid.[3] In addition to protecting the virus’s genetic material, the capsid proteins enable poliovirus to infect certain types of cells. Three serotypes of poliovirus have been identified—poliovirus type 1 (PV1), type 2 (PV2), and type 3 (PV3)—each with a slightly different capsid protein.[9] All three are extremely virulent and produce the same disease symptoms.[3] PV1 is the most commonly encountered form, and the one most closely associated with paralysis.[10] Individuals who are exposed to the virus, either through infection or by immunization with polio vaccine, develop immunity. In immune individuals, IgA antibodies against poliovirus are present in the tonsils and gastrointestinal tract and are able to block virus replication; IgG and IgM antibodies against PV can prevent the spread of the virus to motor neurons of the central nervous system.[11] Infection or vaccination with one serotype of poliovirus does not provide immunity against the other serotypes, and full immunity requires exposure to each serotype.[11]
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