A nationwide outbreak of a respiratory virus last fall sent droves of children to emergency departments.

The infections have now subsided, as researchers knew they would, but they have left behind a frightening mystery.

Since August, 103 children in 34 states have had an unexplained, poliolike paralysis of an arm or leg.

Each week, roughly three new cases of so-called acute flaccid myelitis (AFM) are still reported to the Centers for Disease Control and Prevention.

Is the virus, called enterovirus 68, really the culprit? Experts aren’t certain: Unexplained cases of paralysis in children happen every year, but they are usually scattered and unrelated. After unusual clusters of AFM appeared this fall, enterovirus 68 became the leading suspect, and now teams of researchers are racing to figure out how it could have led to such damage.

“It’s unsatisfying to have an illness and not know what caused it,” said Dr. Samuel Dominguez, an epidemiologist and an infectious disease specialist at Children’s Hospital Colorado, which has had the largest cluster of patients.

For many families, the onset of persistent limb paralysis has been a bewildering experience. Roughly two-thirds of the children with AFM have reported some improvement, the CDC said. About a third show none. Only one child has fully recovered.

In August, Jack Wernick, a first-grader in Kingsport, Tenn., developed a “crummy little cold,” said his father, Dan Wernick, who works for a paper company. It seemed ordinary, until Jack complained that his right arm was heavy, his face began drooping and pain started shooting down his right leg.

On Aug. 29, his parents rushed him to East Tennessee Children’s Hospital. “By then, he couldn’t lift his arm,” Wernick said.

The doctors were perplexed, as his paralysis didn’t quite fit any neurological condition they had seen.

Then Dr. Christopher Miller, a consulting neurologist, read a new CDC advisory describing limb paralysis in children. “The alarm bells went off,” he said.

Today, Jack still cannot lift his right arm; he had not lost use of the affected leg. He is adapting well, but his mother, Nicole Wernick, says she worries that he will never be able to tie his shoes again or drive.

“I’ve run out of tears,” she said.

Parents like her are desperate for scientists to identify the cause. But the research will be slow and painstaking. And while enterovirus 68 is the prime suspect, the CDC has not ruled out another infectious cause, said Mark A. Pallansch, the director of viral diseases.

Jack Wernick and many other affected children have never tested positive for infection with enterovirus 68. Last week, researchers said they could find no enteroviruses or other pathogens in the spinal fluid of 71 children with limb paralysis. “In cases of polio, we routinely find virus in the spinal fluid or the stool,” said Dr. Mary Anne Jackson, the director of infectious diseases at Children’s Mercy Hospital in Kansas City, Mo. She suggested the spine’s gray matter may be damaged not by the virus, but by an immune response to the infection.

If researchers cannot find a virus itself in partly paralyzed children, they can look for its footprint: antibodies to enterovirus 68 in the blood. But first, the CDC had to create an antibody test. “That’s the hard part,” Pallansch said.

For a study at Johns Hopkins University, researchers are enrolling children who have AFM and siblings who were also probably exposed to enterovirus 68 this fall. The scientists hope to compare the genes of children with AFM to others to see if there are differences.

“Are these kids harboring a mutation or a set of mutations that are all the same among kids harboring the disease?” said Priya Duggal, the director of the genetic epidemiology program at Johns Hopkins Bloomberg School of Public Health, who will help lead the study.

At this point, it’s unclear whether paralysis will continue to affect clusters of children. But that possibility makes deducing its cause a priority.

Jack Wernick still needs help to unzip his coat. He used to tell his mother he would be riding his bike in the spring. “He’s stopped saying ‘in the spring,’ ” she said.