A rare, polio-like syndrome that has no known cure has emerged in a small number of children in California, US researchers have said.
Five cases of sudden onset paralysis were described by Stanford University experts at the American Academy of Neurology annual meeting in Philadelphia.
"Although poliovirus has been eradicated from most of the globe, other viruses can also injure the spine, leading to a polio-like syndrome," said Keith Van Haren, Stanford neurologist and lead author of the case.
"In the past decade, newly identified strains of enterovirus have been linked to polio-like outbreaks among children in Asia and Australia," he said in a statement.
"These five new cases highlight the possibility of an emerging infectious polio-like syndrome in California."
Polio has been largely wiped out across the globe, thanks to the introduction of an effective vaccine in the mid-1950s.
However, outbreaks of the highly-contagious disease continue in parts of the world, including Pakistan, Nigeria and Afghanistan.
The children in California tested negative for polio and all had been vaccinated.
The patients showed similar symptoms, such as sudden loss of movement in one or more of their limbs, resulting in paralysis usually within two days.
Three of the five youths had respiratory illness before their symptoms began.
Two have tested positive for enterovirus-68, a rare virus that has been previously associated with polio-like symptoms.
The other three did not test positive and doctors are still searching for the cause of their paralysis.
"We would like to stress that this syndrome appears to be very, very rare," said Mr Van Haren.
However, doctors believe there are likely more cases out there and they are urging parents to contact a doctor if their child shows signs of paralysis.