About three in four polio infections never show any symptoms
Poliovirus infects only humans and spreads mainly by fecal–oral routes—or sometimes mouth to mouth. Roughly 75% of infections stay silent, yet carriers can shed virus for up to six weeks. Permanent paralysis is uncommon; when breathing muscles fail, death can follow.
Mild cases may bring sore throat and fever that pass in one or two weeks; more severe illness can add headache, neck stiffness, and paresthesia. Years later, post-polio syndrome may slowly return muscle weakness resembling the original attack. Diagnosis uses fecal virus or blood antibodies. Ancient art already shows the disease; Michael Underwood recognized it as distinct in 1789, and Karl Landsteiner identified the virus in 1909. Late-nineteenth-century European and U.S. outbreaks made it a feared childhood scourge until 1950s vaccines crashed incidence. As of October 2023, only Pakistan and Afghanistan remained endemic for wild poliovirus. No specific antiviral cure exists; prevention needs multiple vaccine doses for lasting protection.
Two vaccine families dominate: injected inactivated virus (IPV) and oral attenuated live virus (OPV). Together they cut estimated wild cases from about 350,000 in 1988 to 30 confirmed cases in 2022 across three countries. Traditional OPV can rarely revert to virulence; a more genetically stable oral vaccine (nOPV2) gained full WHO licensure and prequalification in December 2023. Circulating vaccine-derived poliovirus emerges when weakened vaccine strains keep spreading in under-vaccinated communities and mutate. Three wild serotypes exist; type 1 is most linked to paralysis. Wild types 2 and 3 were certified eradicated in 2015 and 2019. Immunity to one serotype does not cover the others.
About 25% of infections cause minor temporary symptoms. In roughly 1%, virus reaches the central nervous system—usually as nonparalytic aseptic meningitis. About one to five in 1,000 cases become paralytic acute flaccid paralysis, most often in the legs, sometimes involving head, neck, or diaphragm; among those paralyzed, 2–10% die when breathing muscles fail. Adults more often show symptoms than children. Incubation is about three to six days for nonparalytic disease; paralysis, if it comes, arrives within 7–21 days. After mouth entry via the CD155 receptor, virus multiplies in gut and lymphoid tissue, then blood; CNS invasion seems largely chance. Peak infectiousness spans about a week before and after symptoms, with summer–autumn seasonality in temperate climates.
Source: Polio
