When 4-year-old Sofia Jarvis danced around her father, her right hand holding his over her head as she twirled, she appeared for all the world like a normal, healthy little girl -- until you noticed how her left arm hung limply, tossed slightly by the movement of her body.
Sofia, smiling and laughing, was with her parents and her brothers at a news conference at Lucile Packard Children's Hospital at Stanford University to talk about the mysterious polio-like illness that struck her and has so far robbed her of the use of her arm.
Doctors said that Sofia is one of five children in California showing signs of the illness. The five cases are expected to be presented at the American Academy of Neurology's next annual meeting.
"Most of the kids are still paralyzed. ... There can be some mild improvement but the level of paralysis remains severe," said Dr. Emmanuelle Waubant, a neurologist at the University of California, San Francisco.
She said this evening that doctors still do not know what is causing the illness.
"We're not exactly clear what is the agent," she said. "We suspect it is a virus."
She said they believe it may be a virus that is just a benign cold for most children, but for a very few results in much more serious symptoms.
Sofia's problems began when her parents noticed she was having trouble breathing. After treatment by her pediatrician didn't help, Sofia spent four days in the hospital, but her breathing was still not completely clear.
Her doctor suspected it might be pneumonia and gave her an antibiotic, but as the family was leaving the doctor's office the little girl reached her left hand out for a toy, and, her mother said, "mid-grasp her left hand dropped."
Jessica Tomei said she thought her daughter's arm was hurting because that was where he had an IV, but three days later she still was not using her arm.
This time, her pediatrician prescribed steroids, but again there was no improvement. Then came the weakness in Sofia's left leg and paralysis in her left arm.
That was when they contacted Dr. Keith Van Haren at Lucile Packard Children's Hospital at Stanford University.
"You can imagine. We had two boys that are very healthy and Sofia was healthy until that point," Tomei said. "We did not realize what we were in store for. We did not realize her arm would be permanently paralyzed."
Despite Sofia's paralysis, Tomei said the family feels lucky. The little girl is getting physical therapy, attends pre-school and is learning to write and dances.
"I know that we're so lucky that she's here. We're so lucky," she said. She's going to do amazing things."
Experts have been investigating the illness since 2012, when a doctor sought a polio test for a child. They are currently looking into as many as 25 additional cases.
The children's average age is 12 and all are unable to move some or all of their limbs. Some of the kids first showed symptoms of a common cold.
The children had been vaccinated against polio and tests later confirmed that their illness was not that disease. Polio was eradicated in the U.S. more than 30 years ago.
Researchers suspect the children's illness may be a related virus and they are now asking doctors around the U.S. to look for other cases.
The Centers for Disease Control and Prevention says it is aware of the findings and caution that most polio-like viruses do not result in paralysis.
In all of the cases the UC San Francisco researchers have looked at, the children suffered paralysis in at least one limb, with other limbs less-severely affected.
The researchers said that the illness is still extremely rare, and not in any way a widespread outbreak, but that they have seen more cases recently than they have seen in the past.
They hope to get the word out to physicians so that if they see similar cases they will alert the California Department of Public Health.
The Associated Press contributed to this story.