The Centers for Disease Control and Prevention (CDC) has confirmed a record number of cases of a polio-like illness in 2018.

So far this year, the CDC has confirmed 158 cases of acute flaccid myelitis (AFM), a polio-like illness that primarily strikes children. The CDC is investigating an additional 153 cases.

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That’s the highest number of cases of AFM since the CDC began tracking it in 2014.

The CDC confirmed 22 cases in 2015 and 149 in 2016. In 2017, the CDC confirmed 35 cases.

AFM, a serious condition that affects the nervous system, is still rare, the CDC said.

Most AFM patients had a mild respiratory illness or fever consistent with a viral infection before developing AFM.

The CDC still does not know the cause of AFM, however.

“As a mom, I know what it’s like to be scared for our children and I understand parents want answers,” Dr. Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, said in a press call with reporters last month.

“Right now science doesn’t give us an answer. That’s why we at the CDC and all our partners will keep looking for answers.”

The CDC said last week the number of cases this year seems to have peaked.