Polio (poliomyelitis) is a contagious, acute viral disease caused by poliovirus, a highly infectious enterovirus belonging to the family Picornaviridae. The human is the only natural host for the virus. There are three poliovirus serotypes (type 1, 2 and 3), each of which can cause polio.
The virus is spread person-to-person, typically through faecal-oral route (eg contaminated water, contaminated hands). It enters the body through the mouth, replicates in the intestines and is subsequently excreted in the faeces. During passage through the intestines, it can enter the blood stream to invade the central nervous system, where it spreads along nerve fibres.
Less than 1% of cases result in varying degrees of irreversible paralysis (usually in the legs), and possible death [1,2]. Most infected people have no or very mild flu-like symptoms. Thus infection with the poliovirus usually goes unrecognized.. However, post-polio syndrome may occur years after infection, with a slow development of muscle weakness. The death rate for paralytic polio is 2-5% among children and up to 15-30% for adults .
Polio primarily affects children, with 50% of all cases in children under three years of age. However, persons of all ages can be infected with and potentially transmit poliovirus .
There is no cure for polio and treatment is only symptomatic once the disease is contracted. In the pre-vaccination era, polio was the leading cause of permanent disability. Since the first introduction of polio vaccines in 1955, there has been a dramatic reduction in number of polio cases. Cases due to wild poliovirus have decreased by over 99% since 1988, from an estimated 350,000 cases to 22 reported cases in 2017 . Wild polio virus type 2 was eradicated in 2015 and type 3 in 2019. Today (2020), only two countries in the world have never completely stopped transmission of polio (Pakistan and Afghanistan) 
Oral Polio Vaccine (OPV) can (very rarely) revert to virulence and behave like the wild virus i.e. causing vaccine derived polio. Since 2017, clinical polio cases (caused by both wild type and vaccine derived polio virus) are increasing .
For more details, please see WHO disease description for Polio.
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