Polio is a contagious viral illness that in its most severe form causes paralysis, difficulty breathing and sometimes death.

What is it?

Polio is a contagious viral illness that in its most severe form causes paralysis, difficulty breathing and sometimes death.


Although polio can cause paralysis and death, the vast majority of people who are infected with the poliovirus don't become sick and are never aware they've been infected with polio.

Nonparalytic polio

Some people who develop symptoms from the poliovirus contract nonparalytic polio — a type of polio that doesn't lead to paralysis (abortive poliomyelitis). This usually causes the same mild, flu-like signs and symptoms typical of other viral illnesses.

Signs and symptoms, which generally last two to 10 days, include:

  • Fever
  • Sore throat
  • Headache
  • Vomiting
  • Fatigue
  • Back pain or stiffness
  • Neck pain or stiffness
  • Pain or stiffness in the arms or legs
  • Muscle spasms or tenderness
  • Meningitis

Paralytic polio

Fewer than 1 percent of people infected with poliovirus develop paralytic polio, the most serious form of the disease. Initial signs and symptoms of paralytic polio, such as fever and headache, often mimic those of nonparalytic polio. Between one and 10 days later however, signs and symptoms specific to paralytic polio appear, including:

  • Loss of reflexes
  • Severe muscle aches or spasms
  • Loose and floppy limbs (acute flaccid paralysis), often worse on one side of the body

The onset of paralysis may be sudden.

Classifications of paralytic polio

Paralytic polio has historically been divided into several types, depending primarily on which part of the body is affected. These classifications aren't rigid, and overlap may occur among the different forms.

  • Spinal polio. This most common form of paralytic polio attacks certain nerve cells (motor neurons) in your spinal cord and may cause paralysis of the muscles that control breathing and those in your arms and legs. Sometimes the neurons are only damaged, in which case you may recover some degree of muscle function. But if the neurons are completely destroyed, the paralysis is irreversible, although you still retain your sense of feeling, unlike after many spinal cord injuries.
  • Bulbar polio. In this severe type of polio, the virus affects the motor neurons in your brainstem, where the centers of the cranial nerves are located. These nerves are involved in your ability to see, hear, smell, taste and swallow. They also affect the movement of muscles in your face and send signals to your heart, intestines and lungs. Bulbar polio can interfere with any of these functions but is especially likely to affect your ability to breathe, speak and swallow and can be fatal without respiratory support.
  • Bulbospinal polio. A combination of both bulbar and spinal paralytic polio, this form can lead to paralysis of your arms and legs and may also affect breathing, swallowing and heart function.

Post-polio syndrome

Affecting some people who have recovered from polio, post-polio syndrome is a cluster of disabling signs and symptoms that appears decades — an average of 30 to 40 years — after the initial illness. Common signs and symptoms include:

  • Progressive muscle or joint weakness and pain
  • General fatigue and exhaustion after minimal activity
  • Muscle atrophy
  • Breathing or swallowing problems
  • Sleep-related breathing disorders, such as sleep apnea
  • Decreased tolerance of cold temperatures 


The poliovirus resides only in humans and enters the environment in the faeces of someone who's infected. Poliovirus spreads primarily through the faecal-oral route, especially in areas where sanitation is inadequate.

Poliovirus can be transmitted through contaminated water and food or through direct contact with someone infected with the virus. Polio is so contagious that anyone living with a recently infected person is likely to become infected too. Although people carrying the poliovirus are most contagious 7 to 10 days before and after signs and symptoms appear, they can spread the virus for weeks in their faeces.

Risk factors

You're at greatest risk of polio if you haven't been immunized against the disease. In areas with poor sanitation and sporadic or nonexistent immunization programs, the most vulnerable members of the population — pregnant women, the very young and those with weakened immune systems — are especially susceptible to poliovirus.

These factors also increase your risk if you haven't been vaccinated:

  • Travel to an area where polio is common or that has recently experienced an outbreak
  • Living with or caring for someone who may be shedding poliovirus
  • Handling laboratory specimens that contain live poliovirus
  • A compromised immune system, such as occurs with HIV infection
  • Having had your tonsils removed (tonsillectomy)
  • Extreme stress or strenuous physical activity after being exposed to poliovirus, both of which can depress your immune system


Paralytic polio can lead to temporary or permanent muscle paralysis, disability, and deformities of the hips, ankles and feet. Although many deformities can be corrected with surgery and physical therapy, these treatments may not be options in developing nations where polio is still endemic. As a result, children who survive polio may spend their lives with severe disabilities.


Doctors often recognize polio by symptoms such as neck and back stiffness, abnormal reflexes, and difficulty swallowing and breathing. To confirm the diagnosis, a sample of throat secretions, stool or cerebrospinal fluid — a colorless fluid that surrounds your brain and spinal cord — is checked for the presence of poliovirus.