A new polio-like illness has affected at least 20 children in California over the past 18 months, in an outbreak first identified by Lucile Packard Children’s Hospital pediatric neurologist Keith Van Haren.

In a report on the “new type of acute flaccid paralysis” that will be presented to the American Academy of Neurology’s annual meeting in April, Van Haren notes that the five cases he studied did not regain motor function even after treatment.

While the exact cause of the disease is unknown, researchers suspect it may be caused by a virus, as two of the patients tested positive for enterovirus-68.

Van Haren emphasized to CNN that this new disease is not an epidemic. However, he encouraged parents to immediately take children to a doctor if they exhibit any symptoms of paralysis.