Sofia Jarvis, seen here with her family at a press conference in February, is one of several dozen children in California who have been diagnosed with a rare paralytic syndrome. It has left her left arm paralyzed. Image: Martha Mendoza/AP

When nine children in one hospital in Colorado come down with a mysterious form of paralysis in less than two months, it's hard not to worry.

Colorado has been hit hard by enterovirus D68, a virus that has caused severe respiratory illnesses in children around the country over the past few months.

All of the children had had been ill within a few weeks of suffering weakness in their arms or legs, a drooping face or difficulty swallowing.

But just four tested positive for EV-D68, according to a report published Friday in Morbidity and Mortality Weekly Report.

So it's not certain that the enterovirus is what's causing the paralysis. "There's the association," says Dr. Avindra Nath, clinical director for the National Institute for Neurological Disorders and Stroke and a member of the American Academy of Neurology. "Now the issue is what's the causation."

To find out if the virus is indeed causing the weakness, it would need to be found in the children's spinal fluid, Nath told Shots. So far it's been found only in their noses. Still, "the possibility that this virus may be doing something is certainly worthy of consideration."

The association between paralysis or muscle weakness and EV-D68 first cropped up in California in August 2012, when a 29-year-old man with sudden paralysis was referred for testing for the virus that causes polio. That's also an enterovirus. He didn't test positive for polio or any other enterovirus (there are upwards of 100).

Since then, 23 other cases of "acute flaccid paralysis" of unknown origin have been reported in California, almost all of them in children. Most of the children had an upper-respiratory infection or stomach bug shortly before. Two of the patients had EV-68 in their noses, according to a report also published Friday in MMWR.

Still hearing that paralysis is even just "associated" with upper-respiratory infections in children is enough to make any parent nervous. But these cases are quite rare, Nath says. "There are a lot of respiratory infections, even with this same virus, and the paralysis occurs in a very, very few."

Enterovirus D68 is itself a bit of a mystery. Until this year EV-D68 only rarely caused illness in the United States. Public health types don't know why it's suddenly making many children ill. Some have developed breathing problems and required hospitalization. Many of those children have asthma or wheezing.

Eight Colorado children diagnosed with paralysis are still having problems with weakness, according to the MMWR report. There is no vaccine or antiviral medication for EV-D68. But if a child does appear to have muscle weakness, it's important to take them to a pediatrician or neurologist right away, Nath says.

This sort of paralysis can be caused by a number of things, including Guillian-Barre syndrome and botulism. And even though polio no longer exists in the United States, the MMWR report says that doctors should always test to see if a child with symptoms has been vaccinated against polio, because it's still endemic in parts of Africa and Asia.

The mystery of these new cases of paralysis, and whether they are indeed related to EV-D68, won't be solved until doctors have a clearer sense of the symptoms and how the virus might cause them. If it does prove to be the cause, then someone would need to develop a drug to slow it, or a vaccine to stop it. No small order.

"You've got to start somewhere," Nath says.

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