(CNN) -- Another child in Colorado has been hospitalized with partial paralysis, Children's Hospital Colorado announced Friday.
Doctors don't know what is causing the mysterious neurological illness. They have identified 10 other children at the hospital with similar symptoms: limb weakness, cranial nerve dysfunction and abnormalities in the spinal gray matter. Four children in Boston also match the criteria for this illness, doctors say.
The mother of one of the Colorado children described the family's "scary experience" at a telebriefing on Friday.
It started with a cough. Then the girl became lethargic and her fever rose.
Her mom took her to the doctor, but the girl's blood work looked fine. Doctors sent her home to rest.
The next day, the girl complained of weakness in her arm.
"I thought that she was being dramatic and faking it," her mom said, speaking on the condition of anonymity.
But over Labor Day weekend, the muscle weakness got worse. "She kept dropping her corn on the cob." Almost two weeks after the illness started, the girl was admitted to the hospital.
Some of the children in Colorado and Boston have tested positive for enterovirus D68, a virus that has been sending children around the country to the hospital with severe respiratory illness. But others had no signs of the virus in their system.
"It's probably going to be weeks to months in terms of trying to understand what's happening," said Dr. Samuel Dominguez with Children's Hospital.
As of Wednesday, the Centers for Disease Control and Prevention had confirmed 500 cases of respiratory illnesses caused by enterovirus D68 in 42 states.
Enterovirus D68 has also been found in samples from four people who died recently, according to the CDC, but it's unclear what role the virus played in their deaths. It's possible that enterovirus had nothing to do with them. The virus is very common, especially in the late summer and early fall, with the CDC estimating 10 million to 15 million infections each year in the United States.
"I tend to be a fairly calm, laid-back mom," the girl's mother said when asked about her recommendation for other parents. "Yes, things are mysterious, and it's hard in life when we can't get an exact answer, but I really feel that it's not something that other people should panic about."
Her daughter has returned to school and continues to do physical therapy to regain strength in her am. Several children are still in the hospital, Dominguez said.
"The other kids who have been admitted have continued to be stable ... they're not getting worse but they're not showing dramatic improvement."