As of November 2, there have been 80 confirmed cases of the polio-like sickness known as AFM, or Acute Flaccid Myelitis,  in 25 states. In addition, there are 219 cases under investigation.

The CDC noted an increase in reports of patients under investigation who began experiencing symptoms in August, September and October. It has not identified the 25 states with confirmed illnesses, nor has it said how many states are reporting cases under investigations.

AFM is a rare illness that affects the nervous system. It can cause muscle weakness and sudden onset of paralysis. According to the CDC, 90% of patients since 2014 have been children under the age of 4.

Other symptoms may include: drooping of the face or eyelids, difficulty moving the eyes, difficulty with swallowing or slurred speech. Numbness or tingling may be reported in people with AFM, but it’s often rare. The most severe symptom is respiratory failure that can happen when the muscles involved with breathing become too weak.

According to the CDC, there have been 404 confirmed cases in the U.S. since August 2014. Research is currently underway to determine the cause of AFM. Although, there is a focus on enteroviruses, which can cause respiratory illness and West Nile Virus.

“Even with an increase in cases since 2014, AFM remains a very rare condition. Less than one in a million people in the United States get AFM each year,” the CDC says.