Polio, or poliomyelitis, is an infectious disease caused by polioviruses, members of the Enterovirus genus within the Picornaviridae family. The virus is an enterovirus with a small, non-enveloped RNA genome and is transmitted primarily by the faecal-oral route, although oral-oral transmission can also occur. Most infections are asymptomatic or cause a mild nonspecific illness, but a small proportion invade the central nervous system and can damage motor neurons, producing acute flaccid paralysis. Polio is therefore unusual in that a disease often clinically silent at population level can nevertheless cause severe permanent neurological disability in a minority of infected people.
Poliovirus initially replicates in the oropharynx and gastrointestinal tract and can spread through lymphatic and bloodstream routes. In a small proportion of infections, the virus reaches the central nervous system and infects motor neurons, particularly in the spinal cord and brainstem. Destruction of these neurons produces weakness or paralysis that is typically flaccid rather than spastic during the acute phase. Respiratory muscle involvement can impair breathing and historically led to prolonged dependence on mechanical ventilation. The nervous-system injury explains why paralysis can persist after the virus itself has been cleared.
Clinical disease ranges from asymptomatic infection through minor febrile illness and non-paralytic meningitis to paralytic poliomyelitis. Paralysis can develop rapidly and may affect one or more limbs, although patterns vary. Sensory function is often relatively preserved compared with motor function because poliovirus preferentially damages motor neurons. Bulbar involvement can affect swallowing, speech and respiratory function. Even when a patient survives acute paralysis, residual weakness may remain permanently because mature motor neurons have limited capacity for replacement.
Diagnosis involves clinical assessment combined with laboratory detection of poliovirus or evidence of infection. Stool samples are particularly important for virological surveillance because the virus replicates in the gastrointestinal tract and is shed in faeces. Molecular methods and viral culture can identify poliovirus, while sequencing can help determine whether an isolate is related to a known vaccine-derived or wild-virus lineage. Suspected poliomyelitis is a public-health emergency in many surveillance systems because rapid investigation is essential for detecting transmission and preventing further cases.
Vaccination is the central means of polio prevention. Two major vaccine approaches are the inactivated poliovirus vaccine, which is administered by injection and cannot cause vaccine-derived poliovirus infection because it contains inactivated virus, and oral poliovirus vaccines containing live attenuated strains. Oral vaccines have important advantages for interrupting intestinal transmission because they generate mucosal immunity, but under conditions of insufficient population immunity, vaccine-derived polioviruses can rarely emerge and circulate. Vaccine policy therefore balances individual protection, population transmission control and the epidemiological status of different poliovirus types.
Polio has no curative antiviral treatment that reliably reverses established motor-neuron damage. Management of acute illness is supportive and may involve respiratory support, physiotherapy, nutritional support and treatment of complications. Rehabilitation can help people maximise function after paralysis, although it cannot regenerate destroyed motor neurons. Some survivors develop post-polio syndrome decades after their original illness, involving new weakness, fatigue, muscle pain or reduced endurance. The mechanisms are thought to involve long-term stress on surviving enlarged motor units, but diagnosis requires careful differentiation from other neurological and musculoskeletal causes.
Polio is often confused with conditions that cause paralysis through other mechanisms. Paralytic polio is an infectious motor-neuron disease, whereas many other causes of acute flaccid paralysis have different infectious, immune-mediated, toxic or structural origins. Another misconception is that eradication means the virus can no longer occur anywhere; eradication requires interruption of transmission globally and remains a continuing international objective. Because poliovirus can circulate silently through asymptomatic infections, absence of diagnosed paralysis alone does not prove absence of transmission.
The history of polio control is a major example of the impact of vaccination and coordinated international surveillance. Wild poliovirus transmission has been eliminated from most of the world, but persistent transmission in limited areas and outbreaks involving vaccine-derived polioviruses demonstrate the difficulty of achieving complete global eradication. Environmental surveillance of sewage can detect poliovirus circulation even when no paralytic cases have been identified. Eradication ultimately depends on maintaining high population immunity, rapidly investigating suspected cases, improving surveillance and reaching populations that remain under-immunised or difficult to access.
