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Disease detectives say they’re still stumped by what causes a rare, polio-like paralyzing condition in kids and say they don’t have a No. 1 suspect.

They’ve tested kids with acute flaccid myelitis for more than 250 different viruses and say no single one appears to be a major cause of the condition, which can leave patients disabled for months.

Bailey Sheehan, 8, of Portland, Ore., is one of about 300 kids nationwide who suffered a strange, polio-like muscle weakness once suspected to be linked to a distant relative of polio called EV-D68. Courtesy Sheehan Family

Not only that, but they can’t say who’s more at risk or how cases tend to progress, Tracy Ayers of the Centers for Disease Control and Prevention told a meeting of the CDC’s Epidemic Intelligence Service.

“We have tested for over 250 different organisms that could be causing this,” Ayers told reporters. “We are also expanding to look at non-infectious diseases.”

“After a decrease in 2015, acute flaccid myelitis cases increased during 2016 raising concerns of a resurgence,” Ayers and colleagues wrote in a brief summary.

The CDC says that 138 people in 37 states had the condition in 2016, with five more cases reported so far this year. “Even with an increase in cases in 2016, acute flaccid myelitis remains a very rare disease (less than one in a million),” CDC added.

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Related: Could This Virus Be Paralyzing Kids?

Ayers described the case of one Arizona child who became suddenly paralyzed.

“During the day she was fine but she was gradually not feeling good and by the time it came time to go to bed — the bed hurt,” Ayers said. “Everything hurt to touch.” The mother put the girl into the bath to relieve her pain and became worried when the girl’s head went floppy. She rushed her to the emergency room and the muscle weakness worsened so much that the child was put on a ventilator to help her breathe.

There is no specific treatment for the condition. This child was regaining strength with physical therapy, Ayers said.

“This is such a brand-new disease that we don’t know what the long term outcomes are,” she said.

At first, doctors noted an association with a virus called EV-D68, a normally harmless enterovirus that causes common cold-like symptoms. But now that link is no longer clear, Ayers said.

Related: Children Struggle With Mystery Paralysis

West Nile virus can also cause it, as well as adenoviruses, which also cause common cold-like symptoms.

“EV-D68 specifically is one of the many enteroviruses that we tested for,” she said.

The CDC has appealed to doctors to get blood and spinal fluid samples as soon as possible if there’s suspicion of acute flaccid myelitis. The body can quickly fight off infections, leaving little evidence about what caused the condition.

“We need as much information as possible,” Ayers said.

Symptoms of acute flaccid myelitis include sudden onset of weakness in the arms or legs, as well as drooping facial muscles, including the eyelids, and difficulty moving the eyes. Most patients must be hospitalized and a few have been completely paralyzed. The CDC has not reported any deaths.