Since August, more than 100 children across the country have developed a condition best described in three words no parent ever wants to hear: mysterious, sudden paralysis.

The kids have a median age of 8, and three-quarters of them were previously perfectly healthy. For most, the loss of feeling occurred only on one side of the body.

According to the CDC, in most of the children the paralysis coincided with a nasty cold. That made health workers initially think that the paralysis was caused by a rare virus called enterovirus D68. EV-D68, as it's known, is a severe respiratory virus that was going around at the start of school last year, and because it's a relative of polio, it was thought to be causing the polio-like loss of muscle function.

But now, researchers aren't so sure. Of the 71 paralyzed children whose cerebrospinal fluid was tested, none came back positive for enterovirus. The virus has been found in nasal swabs taken from some of the paralyzed children, but researchers say that doesn't indicate as strong of a link as the spinal fluid would have. Researchers at Children’s Hospital Colorado are currently attempting to determine whether the children suffering from paralysis have elevated levels of enterovirus 68 antibodies.

Mary Anne Jackson, chief of infectious diseases at Children’s Mercy Hospital and one of the first doctors to recognize enterovirus D68, said her hospital has had three cases of paralysis. None of the paralyzed kids had enterovirus D68, however, and none of the hospital's 300 patients who had confirmed enterovirus D68 later became paralyzed.

"Right now we have two scenarios and we have no idea how they're related," Jackson told me.

Nationally, only one of the paralyzed children has fully recovered, and about two-thirds have gotten slightly better.

The existence of an unknown paralytic disease is frightening, but the CDC points out it's yet another reason for parents to get kids up-to-date on vaccinations:

"Although the specific causes of this illness are still under investigation, and causal relationship to EV-D68 has not yet been substantiated," the agency said in a statement, "being up to date on all recommended vaccinations is essential to prevent a number of severe diseases."