The average age of children is about 4, she said, and 90 percent of cases the CDC has been studying since 2014 have involved children 18 or younger.
In some cases patients recover from the paralysis, but in others it continues.
ABC News Videos A girl recovers after contracting acute flaccid myelitis (AFM), a rare but serious condition that affects the nervous system and causes the body's muscles and reflexes to become weak. Some also experience facial droop, difficulty moving the eyes, drooping eyelids, difficulty swallowing, or slurred speech.
Since 2014 there have been almost 400 confirmed cases of AFM across the United States.
At least 62 cases have been confirmed in 22 states this year, and at least 65 additional illnesses in those states are being investigated, according to the Centers for Disease Control and Prevention. Authorities say they are still awaiting confirmed lab results from the CDC. It can start with a fever and respiratory illness; then the sick child suddenly loses the ability to move an arm or leg. One death of a child diagnosed with AFM occurred in 2017.
Experts don't know what causes the disease that can lead to muscle weakness or difficulty breathing. "It seems like they're mostly clustered in the summer, but they can occur throughout the year", said Berbari.
The CDC has tested many different specimens from patients with this condition for a wide variety of pathogens, or germs, that can cause AFM.
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Messonnier said that the CDC has neither a clear idea of who might be at higher risk of developing the disease nor why.
Acute flacid myelitis or AFM is not new, but cases have been on the rise since 2014. So far, 2016 was the worst year, with 149 confirmed cases.
"I want parents to know how hard we're working to try to identify the cause of AFM and when we get information that will help them understand it, we're going to make that information available", Messonnier said.
The agency also said it will post case count updates on its website every week now, a change from the monthly updates it had been giving.
By Tuesday, the total number of cases had reached 386, the CDC said.
"You see your child, who likes to run and play", she said.
"At this moment, we don't know if it's a virus that is coming and producing direct damage of the gray matter in the spinal cord", he said, "or if a virus is triggering immunological responses that produce a secondary damage in the spinal cord". "We recommend seeking medical care right away if you or your child develops sudden weakness of the arms and legs", she said.