(CNN) - A polio-like illness called acute flaccid myelitis (AFM) is affecting children at a higher rate, with cases increasing in even-numbered years.
While polio still exists today, cases have decreased by more than 99% since 1988, according to the World Health Organization.
AFM has been peaking every other year since 2014, according to new information released by the Centers for Disease Control and Prevention.
If the pattern holds, 2020 could be another peak year.
The illness has left some children paralyzed.
Doctors say most patients with the AFM had a mild respiratory illness or fever consistent with a viral infection before they developed AFM.
The CDC doesn’t know why a small number of people develop AFM while most others recover.
Researchers say the differences in each case offer clues as to what may cause the illness.
They hope studying them will lead to new treatment and prevention strategies.
According to the CDC, there were 237 cases of AFM in 2018. In comparison, there were 33 cases in 2019.
Peak year cases were also more likely to affect the upper extremities and come after a fever or respiratory illness.
About 90% of AFM cases have occurred in children in 48 states and Washington, D.C.