Polio is an infectious disease caused by a virus that invades the nervous system. It fueled widespread panic in the early 1950s with thousands of deaths and thousands more cases of paralysis. The viral disease had been referenced in medicine and art for centuries, but scientists suggest that sanitation improvements in the 19th century delayed kids' exposure to poliovirus until later in childhood, when they no longer had maternal antibody protection—leading to more severe illness and higher paralysis rates. In 1952, at the epidemic's peak, there were 21,000 cases of paralysis and more than 3,000 deaths in the US.
The disease, formally called poliomyelitis, comes from the Greek words for gray (polios) and marrow (myelos)—a reference to the spinal cord tissue that causes the disease's characteristic paralysis when it's destroyed. There is no cure for polio. It spreads through consumption of infected stool or via respiratory droplets (from coughing or sneezing). Poliovirus multiplies in the throat and gastrointestinal tract. But if it spreads to the nervous system, it can destroy motor neurons of the anterior horn of the spinal cord and brain stem, causing paralysis.
The first polio vaccines were licensed in the US in 1955 (the inactivated polio vaccine, or IPV, which has no live virus) and 1961 (the oral poliovirus vaccine, OPV, which uses a weakened form of the live virus)—leading to a rapid drop in cases. The last US case of naturally occurring poliovirus was reported in 1979.
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This highly infectious disease is transmitted from one person to another, mainly when we eat virus-laden poop particles. Less frequently, contaminated food or water can transmit the virus, or people can be infected via respiratory droplets (from coughing or sneezing).
About 1 in 200 cases of infection causes paralysis. For paralysis to occur, the virus travels from the throat, intestines, and gut to infect specific cells within the spinal cord and brain stem that control movements. Within hours of invading the nervous system, paralysis can occur. The virus can be present in bodily secretions and stool for multiple weeks post-infection.
Polio has a long history in art and medicine, dating back centuries. It includes a representation of an apparent polio survivor with a shriveled leg in ancient Egyptian art. See milestones in the history of polio in this timeline.
Polio, formally called poliomyelitis, comes from the Greek words for gray (polios) and marrow (myelos)—a reference to the spinal cord tissue that, when infected by the virus, causes paralysis.
Almost all children likely used to contract poliovirus, but scientists suggest that when infants encountered it very early in life—due to poorer conditions and hygiene—they still had some maternal protection from pregnancy or through breastmilk, so the symptoms were more flu-like.
Inactivated polio virus vaccines cannot cause illness because the virus isn't alive. It's been the only polio inoculation offered in the United States since 2000 and provides almost complete protection
The American medical researcher developed the inactivated virus vaccine from 1952 to 1955 and proved that it could prevent disease. He tested the virus on himself and his family in 1954 and then carried out formal clinical trials on more than 1.3 million children.
The live virus vaccine, delivered via oral drops, induces the best protection in the body. Because it includes a live, weakened form of the virus there's an extremely small risk it can sicken recipients and potentially cause paralysis. Recipients also shed the virus in their stool, which can rarely lead to vaccine-derived cases of the virus if the virus mutates. (Some users may experience a paywall.)
Worldwide, the number of cases of polio reported each year from 1980 to 2024 has declined. This graphic depicts regional trends and declines by year.
Michael Underwood, an English physician and surgeon, was the first to describe the disorder, noting it caused disability in the lower extremities in children. Large-scale outbreaks of the disease were not observed until the late 19th century.
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