Polio is caused by a virus that has infected humans for thousands of years, likely circulating quietly among our ancestors long before anyone recognized it as a disease. The poliovirus belongs to a family of gut-dwelling viruses called enteroviruses, and it spread easily through contaminated water and poor sanitation for centuries. What changed wasn’t the virus itself but the world around it. Improved hygiene in the 19th century paradoxically transformed polio from a mild, invisible childhood infection into a terrifying epidemic disease.
The Earliest Evidence of Polio
The oldest known depiction of polio-like paralysis comes from ancient Egypt. A stone carving, or stele, shows a priest named Ruma at the sanctuary of Astarte in Memphis. His leg appears withered and shortened, consistent with the muscle wasting that follows paralytic polio. The carving dates to roughly the 18th dynasty of Egypt, placing it over 3,000 years ago.
Beyond that single artifact, written records are sparse for millennia. Polio almost certainly existed throughout the ancient and medieval worlds, but it rarely left a mark in historical documents. The reason is simple: in societies with poor sanitation, virtually every infant encountered the virus in the first months of life, while still protected by antibodies passed from their mothers. Most were infected, developed immunity, and never showed symptoms. Paralysis was rare enough that no one connected scattered cases into a single disease.
How the Virus Was Identified
The first formal medical description came in 1789, when the English physician Michael Underwood documented a “debility of the lower extremities” in children. But it took more than a century before anyone understood what caused it. In 1908, Karl Landsteiner and Erwin Popper in Vienna proved the culprit was a virus. Their method was elegantly simple: they took spinal cord fluid from a person who had died of polio and passed it through filters fine enough to trap all known bacteria. When the filtered fluid was injected into monkeys, the animals developed polio. Whatever caused the disease was smaller than any bacterium, fitting the new concept of a virus.
The Poliovirus and Its Origins
Poliovirus is a small, hardy pathogen that exists in three distinct types (called serotypes 1, 2, and 3). All three cause identical symptoms, including the paralysis the disease is known for, but they are genetically different enough that immunity to one type doesn’t protect against the others. Each had to be targeted separately in eradication efforts.
The virus evolved from a larger group of enteroviruses, known as species C enteroviruses, that naturally inhabit the human gut. Poliovirus and its close relatives have co-circulated in human populations for a very long time, swapping genetic material through recombination. Research in Madagascar documented this process in action: poliovirus strains and related enteroviruses occupy the same ecological niche in the gut, constantly exchanging segments of their genetic code. The poliovirus genome mutates at a rate of roughly 1% per year, which is fast enough to generate new variants but slow enough that scientists can trace lineages backward through time.
The virus spreads primarily through the fecal-oral route. An infected person sheds enormous quantities of virus in their stool, and in places without clean water or proper sewage systems, it easily contaminates drinking water and food. Less commonly, it can also spread through respiratory droplets or direct oral contact. Once outside the body, poliovirus is remarkably tough. It resists common detergents and can remain infectious for months in cool environments (around 4°C). Heat, chlorine, ultraviolet light, and formaldehyde destroy it, but under the right conditions, contaminated water can spread the virus across entire communities.
Why Epidemics Started in the 1800s
This is the part of polio’s story that surprises most people. For centuries, polio was everywhere but almost invisible. In communities with poor sanitation, babies encountered the virus within their first weeks or months of life. At that age, they still carried protective antibodies from their mothers. The virus would infect them, replicate in their gut, and trigger a lifelong immune response, all while the maternal antibodies prevented the virus from reaching the nervous system and causing paralysis. The result was a population in which nearly everyone was immune by age one.
As sanitation improved in Europe and North America during the late 1800s, this cycle broke. Cleaner water and better sewage systems meant that many children no longer encountered the virus as infants. They grew up without immunity. When they finally did encounter poliovirus, at age five, ten, or even as teenagers, they had no maternal antibodies left to blunt the infection. The virus could replicate freely, and in a small but devastating percentage of cases, it invaded the spinal cord and caused paralysis. The cleaner a country’s water supply became, the older and more vulnerable its population was when outbreaks hit.
This is why polio epidemics were a phenomenon of modernizing, industrialized nations. The massive outbreaks that terrorized the United States and Europe in the early and mid-20th century were not caused by the virus becoming more dangerous. They were caused by children losing the inadvertent protection that poor sanitation had provided for millennia.
Three Strains, Two Eradicated
The global campaign to eradicate polio, launched in 1988, had to eliminate all three types of the virus independently. Wild poliovirus type 2 was declared eradicated in 2015, with its last known case occurring years earlier. Wild poliovirus type 3 followed, with the last detected case in northern Nigeria in 2012 and formal certification of its eradication coming afterward. No wild poliovirus of any type has been detected on the African continent since 2016.
That leaves wild poliovirus type 1 as the only surviving wild strain. It remains confined to two countries: Afghanistan and Pakistan. In 2025, through mid-September, 28 cases were reported (4 in Afghanistan and 24 in Pakistan), a significant drop from 99 cases in all of 2024. Transmission is concentrated along cross-border corridors in southern Afghanistan and northwestern Pakistan, though environmental surveillance continues to detect the virus in sewage samples across all four of Pakistan’s major provinces, including in major cities like Karachi and Lahore.
Vaccine-Derived Polio: A Modern Twist
There is one more chapter in the story of where polio “comes from” today. The oral polio vaccine, which uses a weakened live virus, is the backbone of global eradication efforts because it’s cheap, easy to administer, and spreads immunity through communities. But in places where too few children are vaccinated, the weakened vaccine virus can circulate from person to person long enough to mutate back into a form capable of causing paralysis. These are called circulating vaccine-derived polioviruses.
This isn’t a flaw in the vaccine so much as a consequence of incomplete vaccination campaigns. When coverage is high, the weakened virus encounters immune people and dies out before it can mutate. When coverage drops, the virus has room to circulate, accumulate mutations, and regain its ability to attack the nervous system. Vaccine-derived outbreaks have occurred in parts of Africa, the Middle East, and Southeast Asia, and they represent the other front in the fight to end polio for good.