Acute flaccid myelitis, or AFM, has been compared to polio - a description that strikes fear in the hearts of parents. What starts as a respiratory infection can develop into muscle weakness and paralysis, primarily in children. Sometimes the paralysis is permanent, sometimes not. But investigators don't yet know why, or even what causes the illness.
The first large outbreak of the illness occurred in 2014, during cold and flu season, and during an outbreak of a respiratory virus. Logically, investigators assumed AFM was associated with the same virus. But only about half of patients show evidence of a virus in their respiratory systems.
And when researchers look at spinal fluid - where finding a virus would be a smoking gun for the muscle weakness and paralysis - there's even less to go on.
"We've only found four patients that had evidence of virus, and we found three different viruses," said Tom Clark, an epidemiologist with the Centers for Disease Control and Prevention. "There's nothing definitive yet to pin AFM on a specific viral cause."
What's even stranger is the pattern of AFM's rise over the past five years. There were 120 cases in 2014, 149 in 2016. And that number jumped to 215 confirmed cases in forty states last year. But there were only two or three dozen cases in the intervening years, 2015 and 2017.
"That's new, that's unusual," said Clark. "This every other year pattern, it is a clue. It just doesn't point to anything obvious."
Clark says CDC and academic researchers are continuing to run tests and search for an explanation for AFM. In the meantime, they are trying to improve treatments and get the word out to doctors and parents - so they can be informed, but not afraid.