US doctors are warning of an emerging polio-like disease in California where up to 20 people have been infected.
A meeting of the American Academy of Neurology heard that some patients had developed paralysis in all four limbs, which had not improved with treatment.
The US is polio-free, but related viruses can also attack the nervous system leading to paralysis.
Doctors say they do not expect an epidemic of the polio-like virus and that the infection remains rare.
Polio is a dangerous and feared childhood infection. The virus rapidly invades the nervous system and causes paralysis in one in 200 cases. It can be fatal if it stops the lungs from working.
Global vaccination programmes mean polio is endemic in just three countries - Afghanistan, Nigeria and Pakistan.
There have been 20 suspected cases of the new infection, mostly in children, in the past 18 months,
A detailed analysis of five cases showed enterovirus-68 - which is related to poliovirus - could be to blame.
In those cases all the children had been vaccinated against polio.
Symptoms have ranged from restricted movement in one limb to severe weakness in both legs and arms.
Dr Emanuelle Waubant, a neurologist at the University of California, San Francisco, told the BBC: "There has been no obvious increase in the pace of new cases so we don't think we're about to experience an epidemic, that's the good news.
"But it's bad news for individuals unlucky enough to develop symptoms which tend to be moderate to severe and don't appear to improve too much despite reasonably aggressive treatment."
The cases have been spread over a 100-mile diameter (160km) so the research team do not think the virus represents a single cluster or outbreak.
However, many more people could have been infected without developing serious symptoms - as was the case with polio.
Dr Waubant suspects similar cases in Asia could explain why California is affected, but not the rest of the US.
Fellow researcher Dr Keith Van Haren, from Stanford University, said the cases "highlight the possibility of an emerging infectious polio-like syndrome" in California.
He added: "We would like to stress that this syndrome appears to be very, very rare. Any time a parent sees symptoms of paralysis in a child, the child should be seen by a doctor right away."
Commenting on the findings, Jonathan Ball, a professor of virology at the University of Nottingham, told the BBC: "Since the near-eradication of poliovirus, other enteroviruses have been associated with paralysis, but these viruses usually cause a very mild cold-like illness and severe complications are very rare.
"Two children showed evidence of being infected by a strain of virus called enterovirus-68, which has become strongly associated with outbreaks of respiratory illness.
"Whether or not this strain of enterovirus has caused these or other cases of paralysis is possible but remains conjecture, further studies will be needed to determine this."
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