Acute flaccid myelitis (AFM) is a rare but serious condition that affects the nervous system. People with AFM often experience sudden onset of weakness in their arms or legs and loss of muscle tone and reflexes. Although most patients with AFM have a mild respiratory illness or fever consistent with a viral infection before developing AFM, the cause of AFM in most patients is unknown.
Some viruses, including enteroviruses and West Nile virus, may cause AFM. Oftentimes, however, the cause of a patient’s AFM cannot be determined despite extensive testing. Since 2014, most patients with AFM have had a mild respiratory illness or fever consistent with a viral infection before developing AFM. These types of mild illnesses are common, however, and most people recover and do not develop AFM. The Centers for Disease Control and Prevention (CDC) continues to investigate why a small number of people with respiratory illnesses or fever develop AFM.
Most confirmed cases of AFM have been among children. In 2014, 2016 and 2018, there were increases in AFM cases nationwide, but AFM remains a rare condition. The CDC estimates that fewer than one to two in a million children in the United States get AFM every year.
Symptoms of AFM may include:
- Sudden onset of weakness in the arms or legs
- Loss of muscle tone and reflexes
- Facial weakness or droop
- Drooping eyelids
- Difficulty moving the eyes
- Slurred speech or difficulty swallowing
Rarely, people with AFM may experience numbness or tingling in the limbs or may be unable to pass urine. Severe cases of AFM can involve weakness in the muscles used for breathing, which may cause respiratory failure and require the patient to be placed on a ventilator.
Diagnosis
If you or your child experience symptoms of AFM, seek medical care immediately. Your doctor may perform a variety of tests to try to determine the cause of the symptoms.
Treatment
There is no specific treatment recommended for AFM. It is important for people who experience symptoms of AFM to seek medical care right away so they can be treated on a case-by-case basis. Seeking care early may allow for more prompt diagnosis and testing and helps ensure that patients receive appropriate care if they develop complications.
While the exact cause of AFM is still unknown, you can take steps to reduce the risk of getting viruses that may cause AFM, including:
- Staying up-to-date on vaccinations
- Washing hands frequently with soap and water
- Cleaning and disinfecting frequently touched surfaces
- Avoiding close contact with sick people; and
- Avoiding mosquito bites by using insect repellant, removing standing water near your home, and staying indoors during times when mosquitos are most active (dusk and dawn)
Page last updated/reviewed: April 2026
