KAPP-KVEW has more information on the disease impacting nine children at a Washington hospital.
Two were confirmed to have an illness called "Acute Flaccid Myelitis" or "AFM."
The other seven are still a mystery.
When you ask experts about AFM, there's only so much they can tell you.
"It causes sudden weakness in the arms or legs, sometimes in the face or eye muscles and in severe cases can affect the breathing muscles," said Dr. Jeff Duchin, the public health officer for Seattle and King County.
A Bellingham boy, just 6, passed away on Sunday.
He was one of the nine hospitalized at the Seattle Children's Hospital.
He was never diagnosed with AFM, so his cause of death is still unknown.
"It progressed so fast that it was like you know, kind of right before our eyes," said his aunt Jamie DeGuzman during an interview with KOMO News in Seattle.
"It can be a very serious disease, death is definitely not common with AFM but weakness of the muscles that can persist is not uncommon," said Dr. Duchin.
Two of the illness cases involve children from Franklin County.
"This is most likely an uncommon complication of a common virus that circulates in the community," said Dr. Duchin.
Things like respiratory infections, mosquito borne viruses and even Polio have been linked to AFM.
Dr. Duchin said anyone can get the disease, but it is most common in children under age 18.
He said parents should not be fearful, but cautious, staying away from sickness, using good hand-washing practices and keeping up to date on vaccines so a virus does not turn into AFM.
"We don't know exactly what infection maybe triggering this so we want to protect people from as many types of infections as possible," he said.
Right now there is no treatment for the disease, but doctors are treating symptoms the best they can.
Dr. Duchin said if parents notice muscle weakness in the arms, legs or facial muscles, they should take their kids to the doctor right away especially if they are getting over an illness.
For more information on AFM click here.