The news comes a day after a six-year-old child died, suspected to have been caused by acute flaccid myelitis (AFM). According to the US Centers for Disease Control and Prevention (CDC), "AFM is a condition that affects the nervous system, specifically the spinal cord, which can result from a variety of causes including viral infections.
None of the nine cases share a common infection, illness or other link, said Julie Graham, spokeswoman for the Washington State Department of Health, as reported by the Seattle Times.
"There's nothing that points to an individual cause for any of the cases or a link between the cases," Graham said.
The disease began being tracked in 2014. It is said to mimic polio but is not caused by the polio virus. The disease primarily affects children. The causes of the illness, as well as any medical treatment, are not yet known.
This year, 89 people in 33 states have been confirmed to have the illness, the CDC reported. The increase in the incidence of disease could be accounted for by increased CDC investigation. Though the number of cases has increased, the condition remains extremely rare.