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Medically reviewed by Last updated on Mar 24, 2023.

What is Polio?

Harvard Health Publishing

Polio is a highly contagious infection caused by the poliovirus. Most people infected with the virus develop no symptoms from it. However, in a small percentage of infected people, the virus attacks nerve cells in the brain and spinal cord, particularly the nerve cells in the spinal cord that control muscles involved in voluntary movement such as walking. Permanent paralysis occurs in one out of every 200 cases of polio. Polio is also called poliomyelitis.

The infection spreads through direct contact with virus particles that are shed from the throat or in feces. The disease had been virtually wiped out in the Western hemisphere since the introduction of the inactivated polio vaccine (the "Salk vaccine") in 1955 and the oral, live vaccine (the "Sabin vaccine") in 1961.

Vaccination campaigns have succeeded in reducing the number of countries where polio is endemic (where it occurs locally). In 1988, more than 120 countries contained endemic poliovirus; by 2012, only 3 countries contained endemic polio.

However, in recent years new cases of polio are on the rise, mostly in the developing world where the virus had appeared to have been eradicated. New cases are also arising in Europe and North America, perhaps related to decreased use of recommended childhood vaccines.

In developing countries, some people remain unvaccinated. Poor sanitation and poor hygiene promote the spread of the virus. People traveling to these areas of the world must have up-to-date immunizations.

Though rare, polio symptoms can also happen after people are immunized with the live polio vaccine. Countries that have wiped out polio usually use the inactivated polio vaccine, which never causes polio.


Most cases (about three-quarters) are mild. Symptoms appear three to five days after exposure to the virus and include slight fever, headache, sore throat, vomiting, lack of appetite, and a general feeling of illness and discomfort. The symptoms last for 2-5 days and go away on their own.

In other cases, a more severe illness called paralytic polio develops approximately 7 to 14 days after exposure to the virus. Symptoms include fever, severe headache, stiff neck and back, and deep muscle pain. Some people experience temporary abnormalities of skin sensation. Muscle spasms and a tendency to retain urine are common.

Muscle weakness and paralysis may develop rapidly or gradually during the time fevers occur, but paralysis does not continue to get worse afterwards. The disease most commonly affects the strength of muscles of the legs. It can also affect strength in the muscles of the arms and abdomen. When polio affects strength in the muscles of the neck and throat, it causes difficulty speaking and swallowing. The most life-threatening form of polio causes weakness of the muscles in the chest that are needed for breathing. The virus also can sometimes affect the parts of the brain that control breathing. When a polio victim develops breathing trouble, they may need machines to help breathe for them.


The diagnosis of paralytic polio is based on a neurological exam. Your doctor may suspect that you have polio if you have fever with limb weakness or paralysis that mainly affects one side of your body. Your doctor will test your muscle reflexes and look for muscle weakness, abnormal muscle contractions, and decreased muscle tone. The poliovirus can be detected in throat or stool samples. Antibodies to the virus can be detected in the blood.

Expected duration

Recovery from minor polio occurs in about three days. The fever and other symptoms of paralytic polio can go away within days, but paralysis can be permanent. Some muscle function may return during the first six months after the acute illness, and improvement can continue for two years.


Depending on where they live, infants and children should be immunized with one or both forms of the polio vaccines: the Salk inactivated poliovirus vaccine (IPV), which is given by injection, or the Sabin live attenuated oral polio vaccine (OPV), which is given by mouth. OPV provides the best protection against polio. However, in rare cases, it can cause paralytic polio. For this reason, some countries where polio is no longer endemic now use only the inactivated polio vaccine, which is almost as effective. In the United States, here are the current vaccination recommendations.

For children:

For adults:


No drug can kill the virus once an infection has begun. Treatment is directed at controlling the symptoms of the disease. People with minor poliomyelitis are treated with bed rest and over-the-counter medicines to control fever and muscle aches.

People with major poliomyelitis may require additional treatments, including:

When to call a professional

Seek medical attention for any signs of muscle weakness or paralysis, particularly when accompanied by a fever. Severe headache with stiff neck and back also require medical attention. For people who live in the United States and most other developed nations, such symptoms are unlikely to be caused by poliovirus. However, they could indicate infection with another virus, and always require prompt medical attention.


People with minor illness and nonparalytic forms of polio recover completely, and most people with major illness who were paralyzed also recover completely. Fewer than 25% of people with polio are disabled for life.

Even though you can recover completely from polio symptoms, polio leaves behind some damage. As you age, your nervous system may become less able to compensate for the damage that polio caused, so symptoms may gradually reappear. This can happen 15 or 30 years after the polio infection was active. Recurring symptoms from polio are called post-polio syndrome.

Additional info

Centers for Disease Control and Prevention (CDC)

World Health Organization (WHO)

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.