A mysterious illness causing paralysis in children has been sweeping across the nation. The Centers for Disease Control and Prevention (CDC) has confirmed 62 cases of acute flaccid myelitis (AFM) in 22 states across the nation this year. The CDC is now investigating hundreds of possible cases in multiple states.

AFM is a condition that affects the nervous system. Symptoms include drooping face and eyelids, difficulty moving the eyes and swallowing, and slurred speech. The illness also weakens arms and legs. In severe cases, the affected might have trouble breathing and require a ventilator due to muscle weakness. The symptoms have been described as “polio-like,” but are also similar to the symptoms of West Nile virus and other diseases.

AFM is a very rare disease. The condition affects less than one in a million people in the US annually. In 2014, there were 120 cases of AFM nationwide.

There is no specific treatment for the condition and long-term implications of the illness are unknown. Some patients diagnosed with AFM have recovered quickly. Others continue to have paralysis and require ongoing care.

The CDC said the cause of most AFM cases is unknown. An MRI reveals damage to the gray matter of the spinal cord, and/or changes to the fluid around the spinal cord. Earlier this year, doctors found that AFM could be caused by Enterovirus D68. However, tests of the currently diagnosed cases has been unable to discover a virus that is common to all of them.

Of the 62 confirmed cases, 90 percent are children, with an average age of 4. There is one report of a child dying from AFM in 2017. Parents should seek medical care right away if they or their child develops a sudden weakness or loss of muscle tone in the arms or legs.