Polio is a viral infection that, in its most severe form, destroys nerve cells in the spinal cord and brainstem, causing permanent paralysis. The good news is that most people infected with poliovirus never develop paralysis. Roughly 72% of infections cause no symptoms at all, and about 25% cause only mild, flu-like illness. Less than 1% of infections lead to the paralytic disease most people picture when they hear the word “polio,” but that small percentage is responsible for the devastating outcomes that made polio one of the most feared diseases of the 20th century.
How the Virus Reaches Your Nervous System
Poliovirus spreads through the fecal-oral route, typically via contaminated water or close contact with an infected person. It first replicates in the throat and intestines, which is why most infections feel like a stomach bug or a cold, if they’re felt at all. In rare cases, the virus crosses into the bloodstream and then the central nervous system.
Once in the body, the virus can travel along nerve fibers to reach motor neurons, the nerve cells that control voluntary muscle movement. Research in animal models has shown that poliovirus can be taken up at the junction between nerves and muscles, then transported backward along the nerve fiber toward the spinal cord. The virus rides inside small transport capsules within the nerve without activating until it reaches the cell body of the motor neuron. There, it begins replicating and kills the cell from the inside. The death of these motor neurons is what causes paralysis, and because nerve cells don’t regenerate well, the damage is typically permanent.
What Mild Polio Feels Like
The vast majority of symptomatic polio cases are classified as “abortive” polio, meaning the infection resolves before reaching the nervous system. Symptoms resemble a generic viral illness: fever, sore throat, nausea, headache, and fatigue. These usually last a few days and clear on their own, often without the person ever knowing they had poliovirus.
A smaller number of people develop non-paralytic polio, which affects the lining of the brain and spinal cord (the meninges). This causes a stiff neck, back pain, and headache more intense than the abortive form. It’s uncomfortable but still resolves without permanent damage, typically within about ten days.
What Paralytic Polio Does to the Body
Paralytic polio is the form that causes lasting harm. It begins with the same fever and malaise as mild polio, but within days, muscle weakness sets in and can progress to full paralysis. The weakness is characteristically asymmetric, meaning it tends to hit one side of the body harder than the other, or one limb more than its pair. The legs are affected more often than the arms.
There are three recognized types of paralytic polio, classified by which motor neurons the virus destroys:
- Spinal polio is the most common form, accounting for about 79% of paralytic cases. It targets motor neurons in the spinal cord, causing weakness or paralysis of the limbs, most often the legs.
- Bulbar polio affects the brainstem, which controls muscles used for breathing, swallowing, and facial expression. This was the most dangerous form, accounting for about 2% of paralytic cases. Patients with bulbar polio could lose the ability to breathe on their own.
- Bulbospinal polio combines both, affecting the brainstem and spinal cord simultaneously. It accounted for roughly 19% of paralytic cases and carried the worst prognosis.
Why Some Patients Couldn’t Breathe
The iron lung, the coffin-shaped machine that became a symbol of the polio era, existed because of bulbar and bulbospinal polio. When the virus destroyed motor neurons controlling the diaphragm and the muscles between the ribs, patients lost the ability to inhale and exhale on their own. The iron lung created a sealed chamber around the body from the neck down, using changes in air pressure to pull the diaphragm down and push it back up, mechanically forcing the lungs to fill and empty. Patients had to learn to swallow food in rhythm with the machine’s breathing cycle.
Some patients needed the iron lung only during the acute phase of the illness and eventually recovered enough diaphragm function to breathe independently. Others depended on mechanical ventilation for years, sometimes for the rest of their lives.
Long-Term Effects and Recovery
After the acute infection, some muscle function can return over months as surviving motor neurons sprout new connections to compensate for the ones that were lost. But full recovery from paralytic polio is uncommon. Most survivors live with some degree of permanent weakness, muscle wasting, or limb deformity. Affected limbs may grow more slowly in children, leaving one leg shorter than the other.
Decades after the initial illness, between 25% and 40% of polio survivors develop post-polio syndrome. This typically appears 30 to 40 years after the original infection and involves a new, gradual decline in muscle function. Common symptoms include progressive muscle and joint weakness, unusual fatigue and exhaustion, loss of muscle mass, breathing or swallowing difficulties, sleep apnea, and increased sensitivity to cold temperatures. Post-polio syndrome is thought to occur because the surviving motor neurons that compensated for the damaged ones eventually become overworked and begin to fail. There is no cure, though physical therapy and assistive devices can help manage symptoms.
Where Polio Still Exists
Vaccination has eliminated polio from nearly the entire world. Wild poliovirus type 1, the only wild strain still circulating, is confined to two countries: Afghanistan and Pakistan. As of September 2025, 28 cases of wild poliovirus had been reported globally that year, with 4 in Afghanistan and 24 in Pakistan. The World Health Organization continues to classify polio as a public health emergency of international concern because the virus can spread rapidly in under-vaccinated populations. Occasional detections of vaccine-derived poliovirus in wastewater in other countries serve as a reminder that until transmission is fully stopped, the risk remains.