Updated 3:55 p.m. ET
A rare condition causing weakness in the arms or legs — and sometimes paralysis — has been confirmed in 62 children so far this year, the Centers for Disease Control and Prevention said Tuesday.
One child has died of the condition, called acute flaccid myelitis, or AFM.
At least 65 more cases are under investigation, said Dr. Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Diseases. So far, a common cause linking these illnesses has not been found.
"There is a lot we don't know about AFM," Messonnier said during a teleconference for reporters. "I am frustrated that despite all of our efforts, we haven't been able to identify the cause of this mystery illness."
The average age of the children is about 4, she said, and 90 percent of cases the CDC has been studying since 2014 have involved patients 18 or younger.
Messonnier said scientists don't fully understand the long-term consequences of the illness: "We know that some patients diagnosed with AFM have recovered quickly and some continue to have paralysis and require ongoing care."
Since the condition was first recognized by CDC in 2014, the agency has confirmed 386 cases through Oct. 16, mostly in children. AFM appears to be seasonal, occurring mostly in the late summer and fall, but appears in greater numbers every other year.
The number of cases in 2018 is on track to match a similar number of cases in 2014 and 2016. But Messonnier cautioned that it would be "premature" to be confident that this year will be the same as the earlier years.
It's possible that some milder cases haven't been reported by doctors to their state health department or the CDC, but Messonnier believes that number would be small.
"This is actually a pretty dramatic disease," she said. "These kids have a sudden onset of weakness and they are generally seeking medical care and being evaluated by neurologists, infectious disease doctors and their pediatricians and coming to public health awareness."
Possible causes being considered include viruses that affect the digestive system called enteroviruses, and possibly strains of rhinoviruses, which cause the common cold, she said. The CDC is also considering the possibility that environmental toxins could be triggering the sudden muscle weakness. And it is not ruling out possible genetic disorders.
Media reports in recent weeks have suggested that a "polio-like virus" might be triggering the condition, elevating fears that it might be polio itself.
"Right now, we know that poliovirus is not the cause of these AFM cases," Messonnier said.
She said that CDC has tested every stool specimen from AFM patients. None have tested positive for poliovirus. She also said West Nile virus hasn't been linked to any of these cases, either.
"As a parent myself I understand what it's like to be scared for your child," Messonnier said. "Parents need to know that AFM is very rare, even with the increase in cases that we are seeing now. We recommend seeking medical care right away if you or your child develop sudden weakness of the arms and legs."
Messonnier stressed the rarity of the condition, emphasizing that it happens in fewer than one in a million children in the U.S. So far this year, cases have been confirmed in 22 states, based on findings from MRI studies and the cluster of symptoms a child has.
The CDC says disease prevention steps should be followed, including staying up to date on vaccines, washing hands and using mosquito repellant.