Washington state health officials say a ninth child has been hospitalized with possibly a rare neurological disorder called "Acute Flaccid Myelitis" or AFM at Seattle Children's Hospital.
The Centers for Disease Control is helping local officials determine what is causing the illness.
One child has died, but it's not confirmed if that child died from AFM.
Only two of the nine cases have been confirmed as AFM.
The Washington Department of Health says three children are currently hospitalized and five have been released from the hospital. The children range in age from 3 to 14 years old, and came from four Washington counties: King, Pierce, Franklin and Whatcom.
AFM affects a person's nervous system, specifically the spinal cord. AFM can result from a variety of causes, including viral infections, according to the CDC. Symptoms typically include sudden weakness in one or more arms or legs, loss of muscle ton, and decreased or absent reflexes.
The first eight children treated exhibited a loss of strength or movement in one or more arms or legs.
The health department says many viruses and germs are linked to AFM, including common germs that can cause colds and sore throats, and respiratory infections. It can also be caused by poliovirus and non-polio enteroviruses, mosquito-borne viruses such as West Nile or Zika, and autoimmune conditions.
Washington Department of Health officials have set up a webpage on the AFM investigation.