Polio is a viral disease. Many people contract it and don't have symptoms, and about a quarter of all cases have mild flu-like symptoms. But for a small number of people, polio has severe and life-changing effects on their bodies.
Anyone who has polio can spread the disease, including those who do not have any symptoms. There is no cure for polio, and symptoms can come back decades after the initial infection. All of these factors combined make polio a very dangerous virus.
The polio virus enters the body through the nose and then multiplies in the tonsils and lymph nodes in the neck before traveling to the lymph nodules in the small intestine. The virus takes between two and 35 days to incubate. After three to five days, an infected person starts shedding the virus in their stools, and it is also present in throat swabs.
During this time, most people are asymptomatic or show only mild symptoms, and that is the extent of the infection. For others, the virus spreads into the central nervous system, where it causes extensive damage to motor neurons in the spinal cord and eventual paralysis. The virus can spread further into the nervous system, which can be fatal.
There are various types of polio classified by where the virus attacks. Abortive poliomyelitis is mild, lasting only a few days and causing gastric and flu-like symptoms. Non-paralytic poliomyelitis may cause swelling around the brain—aseptic meningitis—and is more severe than abortive polio.
Paralytic poliomyelitis affects less than one percent of people who get polio. This type happens when the virus attacks the brain and spinal cord, leading to paralysis.
Symptoms vary depending on the type of polio. While many people are asymptomatic, about a quarter of people have flu-like symptoms, including sore throat, fever, headache, nausea, and tiredness. These symptoms usually resolve on their own in two to five days.
Non-paralytic poliomyelitis symptoms include neck stiffness, light sensitivity, pain or tingling in the arms and legs, and severe headache. If the disease progresses to paralytic poliomyelitis, additional symptoms may appear days or weeks later. Symptoms can include limb paralysis or difficulty breathing, speaking, and swallowing, depending on what part of the nervous system is affected. If the virus attacks the brain, symptoms include anxiety, extreme fatigue, and seizures.
Polio is highly contagious. The virus spreads through droplets from sneezing or coughing or if someone comes in contact with the feces of an infected person. Contaminated water can also spread polio, whether you drink it or get some in your mouth, eat foods that have been in contact with it, or swim in it.
Because there is no cure for polio, treatment focuses on symptom management. Early on, treating symptoms involved interventions like hot packs to relieve muscle spasms and preventing affected limbs from being immobilized.
President Franklin Roosevelt opened a rehabilitation center for polio patients in 1926, and technology eventually brought about interventions like the iron lung machine, which helped people with respiratory paralysis breathe. Modern treatments include physical therapy to move paralyzed muscles and orthotic devices to prevent deformities.
Vaccines are the most effective way to prevent polio. There are two types. The inactivated poliovirus vaccines are given as an injection in the arm for older children and adults and in the leg for infants. The U.S. has only used this type of vaccine since 2000.
The second is an oral vaccine and is widely used in other places around the world because it is much easier to distribute and administer. Polio vaccines significantly reduce the risk of getting polio, but they do not eliminate it altogether. Vaccinated people who do get polio usually have milder symptoms.
The U.S. no longer uses the oral polio vaccine because of the risk of vaccine-derived polio. Over time, the live virus used in the oral polio vaccine can mutate and spread through the unvaccinated. This mutated virus acts like naturally occurring polio, making people sick.
Live vaccines are generally not used in areas where the natural virus has been eliminated, significantly lowering the risk of vaccine-derived polio spreading. This form of polio can only spread in areas with low vaccination rates.
Unvaccinated people are the most at risk for getting polio, but some vaccinated people are at higher risk than others. You may need a booster if you are traveling to areas of the world where natural polio still exists or if you work in a lab or in a facility with patients where you may come into contact with the poliovirus.
Most people recover from polio within a week, but it is impossible to tell which cases will be mild and which will progress to paralytic polio. Of those who do get paralytic polio, complications are fatal for about 10 percent of patients.
About 40 percent of people who had polio as children develop post-polio syndrome, which can happen between 15 and 40 years after the initial infection. Symptoms may start mild and worsen over time. They include muscle weakness, difficulty breathing or swallowing, muscle atrophy, scoliosis, and joint, bone, and muscle pain.
In September 2022, an unvaccinated adult in New York state developed paralytic polio, and scientists found the virus in wastewater samples in communities near the patient's home. These two factors show that polio is still circulated in this area of the state and, most likely, in the surrounding regions.
Scientists linked the viral samples to wastewater samples collected in Jerusalem and London, indicating that community transmission is occurring. The samples also indicated that this strain is most likely a vaccine-derived poliovirus and adds the U.S. to a list of about 30 countries where vaccine-derived polio is circulating.
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