A mysterious illness thats sickening and even paralyzing children is prompting warnings this week from federal health officials to seek medical care immediately should anyone develop its symptoms.

Acute flaccid myelitis (AFM), an extremely rare but serious condition that affects the nervous system and causes the bodys muscles and reflexes to become weak, is on the rise across the country, the Centers for Disease Control and Prevention said on Tuesday.

Despite that alarm, there are a lot of unknowns about the polio-like illness, including what causes it and why it affects more children at the average age of 4 than adults.

More than 90 percent of the cases are in children age 18 years and younger, Nancy Messonnier, the director of the National Center for Immunization and Respiratory Diseases, said at a CDC press conference .