Eight of nine children hospitalized in Washington state for a polio-like illness have a rare syndrome that causes varying degrees of limb weakness, state health officials confirmed last week.

The eight cases from five counties in the state were determined by the Centers for Disease Control and Prevention to be acute flaccid myelitis, also known as AFM, which affects the nervous system and specifically the spinal cord.

A ninth child who died this week at Seattle Children’s Hospital, a 6-year-old boy from Bellingham, was found not to have the syndrome and officials are still investigating his cause of death, Washington State Department of Health spokeswoman Julie Graham told The Associated Press.

“He is absolutely not an AFM case,” Dr. James Owens, of Seattle Children’s Hospital, said of the boy at a news conference on Friday.

State officials said the children with AFM, ranging in age from 3 to 14, arrived at Seattle Children’s with their symptoms and did not acquire AFM at the hospital. The children were admitted with a range of types and severity of symptoms, but all had a loss of strength or movement in one or more arms or legs, which is a main symptom of AFM, according to Dr. Scott Lindquist, state epidemiologist for communicable diseases.

Owens said recovery is variable although the symptom of weakness peaks in the first day or so and doesn’t worsen over time.

“In general, none of the children have a life-threatening condition at this point,” Owens said.

No further details were given about the children’s cases.

Health officials said they haven’t found a common link between the children that could explain the cluster of cases. Doctors have emphasized that the syndrome is not contagious.

Scientists at the CDC are working to determine the exact cause of AFM. Many viruses and germs are linked to it, including germs that can cause colds, sore throats and respiratory infections. It can also be caused by poliovirus and non-polio enteroviruses, mosquito-borne viruses such as West Nile virus or Zika virus, and autoimmune conditions.

In 2015, zero cases of AFM were reported in Washington State, and in 2014 there were two. Since September 2016, state officials say there have been 89 cases of AFM in 33 states across the U.S.

The best way to prevent AFM is to keep general illnesses at bay through hand-washing and vaccinating children against the flu, Owens said