Since late last summer and early fall, some 100 or more children across 34 states have fallen ill with a strange new sickness recently named acute flaccid myelitis. Though some of these children show signs of the enterovirus, doctors still aren’t sure whether the two illnesses are related.

The main symptoms of acute flaccid myelitis involve limb weakness and even paralysis. Most of the 100 children infected with the illness haven’t shown improvement, and only one has recovered completely.

“Since September 2014, [the Centers for Disease Control and Prevention] CDC and partners have been investigating reports of children across the United States who developed a sudden onset of weakness in one or more arms or legs with MRI scans that showed inflammation of the gray matter — nerve cells — in the spinal cord,” according to the CDC. “This illness is now being referred to as acute flaccid myelitis.”

Most of the children who had fallen ill with acute flaccid myelitis experienced respiratory problems in the weeks prior, such as coughing and difficulty breathing, followed by a fever and weakness in the limbs. Finally, paralysis symptoms began appearing. The average age of the affected patients is around 7.6 years old, according to the CDC, and almost all of them were hospitalized.

Some of the kids infected showed signs of the enterovirus 68 — a respiratory infection that attacked children last fall — but 71 of them had spinal fluid devoid of any enterovirus.

“It’s unsatisfying to have an illness and not know what caused it,” Dr. Samuel Dominguez, an epidemiologist and infectious disease specialist at Children’s Hospital Colorado (which saw most of the first cases), told The New York Times. If it’s not the enterovirus 68, it’s possible that the virus is simply one that doctors haven’t discovered yet.