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There have been 50 cases of Acute flaccid myelitis in the US up to August this year compared to 21 in the whole of 2015 Flickr/Caroline

A disease similar to polio causing paralysis in children is on the rise in the US – with twice as many affected so far this year as in the whole of 2015 – leaving scientists mystified.

Acute flaccid myelitis (AFM), which causes extreme muscle weakness and paralysis, is up significantly, according to the US Centers for Disease Control.

AFM is characterized by a sudden weakness in one or more arm or leg, along with loss of muscle tone and decreased or completely absent reflexes.

In some cases, facial weakness including difficulty swallowing or drooping eyes may also accompany weakness in the limbs.

Despite extensive testing, the CDC does not yet know the cause of the AFM cases.

AFM appears to start with a fever or respiratory illness before neurological symptoms set in, slowly preventing movement of the limbs.

Nearly all of the patients show inflammation of the grey matter in the spinal cord. Of those who contract the disease, 85% of children show some improvement in their condition. But very few fully recover.

As of August 2016, there have been 50 cases of confirmed AFM across 24 states, according to the CDC. That's more than double the 2015 figures, when 21 cases were reported for the entire year.

AFM has been linked to a strain of enterovirus that's now circulating. Some experts believe this could be the same mysterious, polio-like illness detected in 2014 that paralyzed 120 children.

Enteroviruses usually cause mild illnesses such as colds. But if they infect the central nervous systems, it can be serious.

"August to October is typically when enteroviruses circulate," said Dr Kevin Messacar, pediatric infectious disease physician at the Children's Hospital Of Colorado. "We see more Acute flaccid myelitis during that season and we do seem to be seeing an increase in the cases that have been reported," he told NBC News.

"Acute flaccid myelitis is a very rare complication of an illness. I think it's important that we take it seriously because the effects of this condition appear to be long term and are disabling."

Experts don't know why incidents are up, nor is there a vaccine for it.

For now physicians advise good hygiene to stop it from spreading.