Poliomyelitis, commonly shortened to polio, is an infectious disease caused by the poliovirus.[1] Approximately 70% of cases are asymptomatic; mild symptoms which can occur include sore throat and fever; in a proportion of cases more severe symptoms develop such as headache, neck stiffness, and paresthesia.[1][3] These symptoms usually pass within one or two weeks.[1] A less common symptom is permanent paralysis, and possible death in extreme cases.[1] Years after recovery, post-polio syndrome may occur, with a slow development of muscle weakness similar to that which the person had during the initial infection.[2]
Polio occurs naturally only in humans.[1] It is highly infectious, and is spread from person to person either through fecal-oral transmission[1][5] (e.g. poor hygiene, or by ingestion of food or water contaminated by human feces), or via the oral-oral route.[1] Those who are infected may spread the disease for up to six weeks even if no symptoms are present.[1] The disease may be diagnosed by finding the virus in the feces or detecting antibodies against it in the blood.[1]
Poliomyelitis has existed for thousands of years, with depictions of the disease in ancient art.[1] The disease was first recognized as a distinct condition by the English physician Michael Underwood in 1789,[1][6] and the virus that causes it was first identified in 1909 by the Austrian immunologist Karl Landsteiner.[7][8] Major outbreaks started to occur in the late 19th century in Europe and the United States,[1] and in the 20th century, it became one of the most worrying childhood diseases.[9] Following the introduction of polio vaccines in the 1950s polio incidence declined rapidly.[1]
Once infected, there is no specific treatment.[3] The disease can be prevented by the polio vaccine, with multiple doses required for lifelong protection.[3] There are two broad types of polio vaccine; an injected vaccine using inactivated poliovirus and an oral vaccine containing attenuated (weakened) live virus.[1] Through the use of both types of vaccine, incidence of wild polio has decreased from an estimated 350,000 cases in 1988[3] to 6 confirmed cases in 2021, confined to just three countries.[10] There are rare incidents of disease transmission and/or of paralytic polio associated with the attenuated oral vaccine and for this reason the injected vaccine is preferred.[11]
The term "poliomyelitis" is used to identify the disease caused by any of the three serotypes of poliovirus. Two basic patterns of polio infection are described: a minor illness which does not involve the central nervous system (CNS), sometimes called abortive poliomyelitis, and a major illness involving the CNS, which may be paralytic or nonparalytic.[12] Adults are more likely to develop symptoms, including severe symptoms, than children.[1]
In most people with a normal immune system, a poliovirus infection is asymptomatic. In about 25% of cases, the infection produces minor symptoms which may include sore throat and low fever.[3] These symptoms are temporary and full recovery occurs within one or two weeks.[1][3]