Poliomyelitis

Polio is caused by poliovirus, which multiplies in the throat and intestine. It passes from person to person mainly through the faecal–oral route, including when food or water is contaminated; less commonly, transmission occurs through shared sources. Many infected people have no visible symptoms. Others may develop a short illness with fever, fatigue, headache, vomiting, neck stiffness or limb pain. In a small proportion of infections, the virus affects the nervous system and damages motor neurons, producing weakness or acute flaccid paralysis. Paralysis can be irreversible and, if breathing muscles are affected, life-threatening. Estimates of paralysis risk vary by poliovirus type and source: WHO gives a general estimate of about one infection in 200 leading to irreversible paralysis, while the US Centers for Disease Control and Prevention gives a range of about one in 200 to one in 2,000 depending on the type.

There is no specific cure for polio; vaccination is the principal means of prevention. The two vaccine types are inactivated poliovirus vaccine (IPV), given by injection, and oral poliovirus vaccine (OPV), which contains weakened live virus and is given by mouth. Their use is selected according to public-health conditions and national programmes. OPV can help build intestinal immunity and interrupt transmission, but in populations with low vaccination coverage its weakened virus can rarely change over time and circulate as vaccine-derived poliovirus. High immunization coverage protects people from paralysis and reduces the chance that either wild or vaccine-derived virus will continue to spread. Rehabilitation, including physical or occupational therapy, can help some people with weakness after infection. Some survivors later develop post-polio syndrome, a non-contagious return of symptoms such as fatigue, pain or muscle weakness decades after the original illness.

The World Health Assembly launched the Global Polio Eradication Initiative (GPEI) in 1988. The World Health Organization reports that cases caused by wild poliovirus have fallen by more than 99% since then, through vaccination campaigns, routine immunization and surveillance. Two of the three wild poliovirus types have been declared eradicated: type 2 in 2015 and type 3 in 2019. These milestones do not mean that all poliovirus has disappeared: wild type 1 and vaccine-derived viruses remain relevant to public-health efforts. Because poliovirus can circulate silently among people without symptoms, surveillance and vaccination coverage remain important even where paralytic cases are rare.