Two years after the last outbreak, doctors are seeing another rise in a mystery illness that renders infected children paralyzed.

So far this year, doctors have seen 50 reported cases of acute flaccid myelitis (AFM) in 24 states. This is more than double the total in 2015, which was 21, reports Today.

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AFM is similar to polio in that it affects the spinal cord, but doctors are unsure of the exact trigger and no specific treatment currently exists.

Doctors believe it can be caused by a variety of germs that cause milder illnesses in children, like respiratory infections. When these germs, like enteroviruses, get into the nervous system, they can cause serious illnesses like inflammation of the brain, according to Today.

Early stage symptoms of AFM include limb weakness, facial droop, difficulty breathing or swallowing, and slurred speech.

The best way to prevent spread of the illness is to consistently wash hands and keep children up-to-date on vaccines. Parents should also protect children from mosquito bites since the mosquito-borne West Nile virus is linked to AFM, according to Today.

Now that more cases are popping up, doctors are keeping a close eye on the spread of the disease. At its height in 2014, there were 120 reported cases in children between August and December.