It's only when 4-year-old Sofia Jarvis walks that you can tell something is wrong with her left arm.
It's been paralyzed from a polio-like condition.
Sofia is one of five confirmed cases of the mystery illness.
20 more are suspected.
Doctors say there's no known link between patients.
The illness can start with cold or flu-like symptoms and paralyzes at least one limb.
"The prognosis that we've seen so far is not good. Most of the children we've seen so far have not recovered use of their arm or leg," says Dr. Keith Van Haren.
Doctors suspect a virus is to blame, and say that while frightening, they want to reassure parents this is very uncommon.
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