US health officials are investigating nine cases of muscle weakness or paralysis in Colorado children and any possible link to a widespread virus causing severe respiratory illness.
The Centers for Disease Control and Prevention (CDC) sent doctors an alert on Friday about the polio-like cases and said the germ, enterovirus 68, was detected in four out of eight sick children. The status of the ninth case is unclear.
The virus can cause paralysis but other germs can, too.
Health officials don't know if the virus caused the symptoms.
"That's why we want more information," said the CDC's Dr Jane Seward.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
All nine children fell ill within the last two months and are being treated at the Children's Hospital Colorado.
Each contracted a fever and respiratory illness about two weeks before developing limb weakness.
Investigators don't think it's polio - eight of the nine children are up to date on polio vaccinations.
The cases come amid an unusual wave of severe respiratory illness from enterovirus 68.
The germ was first identified in 1962 and has caused clusters of illness before.
This year, the virus has been linked to hundreds of severe illnesses.
A flood of sick children began to hit hospitals in Kansas City, Missouri, and Chicago beginning last month.
The CDC has tested a number of specimens from very sick children and has reported 277 people with enterovirus 68.
So far no deaths have been attributed to the virus, but Seward said 15 still are being investigated.
Enterovirus can cause paralysis: published reports show at least two US children with paralysis who died were found to have the virus in their spinal fluid.
Earlier this year, Stanford University researchers said they had identified polio-like illnesses in about 20 California children over about 18 months. Two tested positive for enterovirus 68.
CDC officials say it's still not clear if the virus was a factor in those cases.
Dr Larry Wolk, chief medical officer of the Colorado Department of Public Health and Environment, said paralysis was a rare complication of this infection.
But he said as so many more cases of enterovirus were being reported this year, it might not be surprising to see that complication.
