STANFORD (AP) — More than a dozen children in California have developed an extremely rare, polio-like syndrome within the past year that within days paralyzed one or more of the children’s arms or legs, Stanford University researchers say.

The illness is still being investigated and appears to be very unusual, but Dr. Keith Van Haren at Lucile Packard Children’s Hospital at Stanford University warned Monday that any child showing a sudden onset of weakness in their limbs or symptoms of paralysis should be immediately seen by a doctor.

“The disease resembles but is not the same as polio,” he said. “But this is serious. Most of the children we’ve seen so far have not recovered use of their arm or their leg.”

But doctors are not sure if it’s a virus or something else, he said. Dr. Van Haren says he’s studied five cases from Monterey up through the San Francisco Bay Area, including two who were identified with the disease enterovirus-68, which is from the same family as the polio viruses. He said there have been about 20 cases statewide.

“We want to temper the concern, because at the moment, it does not appear to represent a major epidemic but only a very rare phenomenon,” he said, noting similar outbreaks in Asia and Australia.

But for some children, like Sofia Jarvis, 4, of Berkeley, rare doesn’t mean safe.

She first developed what looked like asthma two years ago, but then her left arm stopped moving, and has remained paralyzed ever since.

“You can imagine. We had two boys that are very healthy and Sofia was healthy until that point,” said her mother Jessica Tomei. “We did not realize what we were in store for. We did not realize her arm would be permanently paralyzed.”

Dr. Van Haren, who diagnosed Sofia, said polio vaccines do not protect children from the disease, but stressed that it is still important for children to receive that vaccine.

Dr. Jane Seward of the Centers for Disease Control and Prevention in Atlanta said Monday that the research is still underway in California, and there are a variety of infectious diseases that can cause childhood paralysis.

So there could be any of a number of illnesses at work here, and it’s possible some of the cases had one infection and some had another. Regarding the presence of EV-68 in at least two cases, “it could be an incidental finding,” Seward said. Until they get more information, Dr. Seward said they are not looking around the country for similar cases of EV-68.

UC San Francisco neurology professor Emmanuelle Waubant said they believe, but don’t have proof, that it’s a virus that for most children shows up only as a benign cold. She said a few children, due to their biological makeup, are having much more serious symptoms and she hoped doctors would look for them.

“For a lot of the neurologists who have trained in the last 30 years, it’s extremely rare to see polio or polio-like syndrome,” she said.