Yes, polio is caused by a virus. The disease, formally called poliomyelitis, results from infection with poliovirus, a small, single-stranded RNA virus in the family Picornaviridae. It was first identified in 1908, when Karl Landsteiner and Erwin Popper successfully transmitted the disease from a human to an ape in Vienna, confirming it was caused by what Landsteiner called a “filterable micro-organism,” the term used for viruses at the time.
What Kind of Virus Is Poliovirus?
Poliovirus is classified as a human enterovirus, meaning it primarily infects through the digestive tract. It belongs to the Picornaviridae family, a group of very small, non-enveloped viruses. Its genetic material is a single strand of RNA, not DNA, which it uses to hijack human cells and make copies of itself. The outer shell of the virus is built from 60 copies each of four different structural proteins, arranged into a compact, spherical capsid.
There are three types of poliovirus (types 1, 2, and 3). Wild type 2 was declared eradicated in 2015, and wild type 3 in 2019. Only wild poliovirus type 1 still circulates naturally, limited to Afghanistan and Pakistan as of 2024.
How Poliovirus Spreads
Poliovirus spreads primarily through the fecal-oral route, especially in areas with poor sanitation. This can happen person to person through contaminated hands, water, food, or shared surfaces. A secondary route is oropharyngeal transmission, essentially through respiratory droplets or saliva, which plays a relatively greater role in settings with better sanitation. The virus is remarkably durable in the environment and has been found in sewage, on floors, on the hands and bodies of infected people, and even on flies.
What Happens After Infection
The vast majority of poliovirus infections produce no symptoms at all. About three-quarters of people who are infected never know it. Roughly 24% experience mild, flu-like symptoms: low-grade fever, sore throat, and general malaise that resolves on its own.
The serious damage happens in about 1% of cases. In these people, the virus enters the bloodstream and crosses into the central nervous system, where it targets and destroys motor neurons, the nerve cells that control voluntary muscle movement. This destruction causes the sudden onset of paralysis known as acute flaccid paralysis. The virus can reach the central nervous system either by crossing the blood-brain barrier or by traveling along nerve fibers themselves through a process called retrograde axonal transport, essentially hitchhiking backward along the nerve pathways that connect muscles to the spinal cord.
The paralysis can be temporary or permanent, depending on how many motor neurons are destroyed. When the virus affects the neurons controlling the muscles used for breathing, it can be fatal.
How Poliovirus Is Detected
If a doctor suspects polio, the gold standard for diagnosis is growing the virus from a stool sample in a laboratory culture. Stool is the most reliable source because the virus replicates heavily in the intestinal tract. The CDC recommends collecting at least two stool samples, taken 24 hours apart and ideally within 14 days of symptom onset. Once the virus is isolated, lab technicians use genetic testing to determine whether it’s a wild strain or a vaccine-related strain, and genome sequencing can pinpoint where the virus likely originated geographically.
Two Vaccines, Two Approaches
Two types of polio vaccine exist, each with distinct strengths. The inactivated poliovirus vaccine (IPV) is given by injection and uses a killed version of the virus. It provides strong protection against paralytic disease but generates a weaker immune response in the gut, meaning a vaccinated person can still carry and shed the virus without getting sick themselves. IPV is the standard in the United States and most high-income countries.
The oral poliovirus vaccine (OPV) uses a live but weakened virus and is swallowed. It triggers robust immunity in the intestinal lining, which makes it more effective at stopping the virus from spreading through communities. This makes OPV the preferred tool in mass vaccination campaigns aimed at eradication. However, it carries a rare but significant drawback.
Vaccine-Derived Poliovirus
Because OPV contains a live, weakened virus, the vaccine strain can sometimes mutate as it passes from person to person in communities with very low vaccination rates. Over time, these mutated strains can regain the ability to cause paralysis, creating what’s known as circulating vaccine-derived poliovirus (cVDPV). This is not a risk of the vaccine itself to the person who receives it; it’s a consequence of the weakened virus circulating for too long among unvaccinated people.
The problem is far from theoretical. Between January 2023 and June 2024, 74 cVDPV outbreaks were detected across 39 countries, causing 672 confirmed cases of paralytic polio. The overwhelming majority were type 2, and in 15 countries, transmission persisted for more than a year. This paradox, a vaccine strain causing the very disease it was designed to prevent, is one of the central challenges in the final push toward global eradication.
Post-Polio Syndrome
Even people who recovered from paralytic polio decades ago can face a second round of problems. Post-polio syndrome typically emerges 30 to 40 years after the original infection, bringing new muscle weakness, fatigue, and loss of function. It only affects people who had polio, and the symptoms can be gradual enough that they’re mistaken for normal aging. The syndrome is thought to result from the long-term overwork of surviving motor neurons, which have been compensating for the ones destroyed by the original infection for decades.