Diseases & Conditions



Polio (poliomyelitis, infantile paralysis) is a highly contagious, sometimes fatal, viral infection that affects nerves and can produce permanent muscle weakness, paralysis, and other symptoms.

Polio is caused by poliovirus, an enterovirus, which is spread by swallowing material contaminated by the virus. The infection spreads from the intestine to the parts of the brain and spinal cord that control the muscles.

In the early 20th century, polio was widespread throughout the United States. Today, because of extensive vaccination, polio outbreaks have largely disappeared, and most doctors have never seen a new polio infection. The last case of wild poliovirus infection in the United States occurred in 1979. The Western Hemisphere was certified polio-free in 1994. A global polio eradication program is under way, but cases still occur in sub-Saharan Africa and southern Asia. Unimmunized people of all ages are susceptible to polio. In the past, polio outbreaks occurred mainly in children and adolescents, because many older people had already been exposed to the virus and developed immunity.

Symptoms and Diagnosis

Fewer than 1 out of 60 to 100 infected people develop any symptoms. Of those with symptoms, 80 to 90% simply have fever, mild headache, sore throat, vomiting, and a general feeling of illness (malaise). These symptoms develop 5 to 9 days after exposure to the virus. The remaining 10 to 20% of people have more serious symptoms (major poliomyelitis). Major poliomyelitis is more likely in older children and adults. The symptoms, which usually appear 7 to 14 days after infection, include fever, severe headache, a stiff neck and back, and deep muscle pain. Sometimes areas of skin develop odd sensations, such as pins and needles or unusual sensitivity to pain. Depending on which parts of the brain and spinal cord are affected, the disease may progress no further, or weakness or paralysis may develop in certain muscles. The person may have difficulty swallowing and may choke on saliva, food, or fluids. Sometimes fluids go up into the nose, and the voice may develop a nasal quality. Sometimes the part of the brain responsible for breathing is affected, causing weakness or paralysis of the chest muscles. Some people are completely unable to breathe.

A doctor can diagnose polio by its symptoms. Diagnosis is confirmed by identifying poliovirus in a stool sample or from a throat swab and by detecting high levels of antibodies to the virus in the blood.


Polio vaccine is included among the routine childhood immunizations (see Vaccinating Infants and Children ). Two types of vaccine are available worldwide: an inactivated poliovirus vaccine (Salk vaccine) given by injection and a live poliovirus vaccine (Sabin vaccine) taken by mouth. The live oral vaccine provides better immunity in a population but can mutate and cause polio in about 1 out of every 2.4 million children. Although this is very uncommon, because polio has been eradicated in the United States, doctors recommend only the injected vaccine for children in this country. The oral vaccine is no longer available in the United States but is used in other parts of the world.

A first vaccination of people older than 18 is not routinely recommended because the risk of acquiring polio as an adult is extremely low in the United States. Adults who have never been vaccinated and who are traveling to an area where polio is still a health risk should receive the injected vaccine. Local and state health departments have information about which areas have polio.

Prognosis and Treatment

People with minor polio completely recover. About two thirds of people with major polio have some permanent weakness. Some people, even those who apparently have recovered completely, develop a return or worsening of muscle weakness years or decades after an attack of polio. This condition (postpolio syndrome) often results in severe disability (see Peripheral Nerve Disorders: Postpolio Syndrome ).

Polio cannot be cured, and available antiviral drugs do not affect the course of the disease. A ventilator may be needed if the muscles used in breathing are weakened. Often, the need for a ventilator is temporary.

Last full review/revision May 2007 by Mary T. Caserta, MD

Source: The Merck Manual Home Edition