No further details have been released about the "clinically diagnosed" cases, but Illinois authorities are investigating whether there might be a cluster.
Two girls -- a 2-year-old from Batavia and an 8-year-old from Chesterton, Indiana -- were being treated this week for the illness, also called AFM, at Lurie Children's Hospital in Chicago.
RELATED: Batavia toddler treated for rare polio-like illness in Chicago
The nine cases have not been confirmed by the Centers for Disease Control & Prevention, and none have been confirmed by the CDC this year. However, local officials are working with CDC investigators.
AFM mimics cold or flu symptoms at first and then includes limb weakness and partial paralysis. The illness, which has affected children across the U.S., affects lungs and nervous system.
Only four cases of AFM have been confirmed by the CDC in Illinois since 2015.
There have been at least six cases reported in Minnesota in the last few weeks, and at least 38 confirmed cases in 16 states.
The disease is exceedingly rare.
"As near as we can tell the rates are about one in a million of people getting it," said Dr. Allison Bartlett, University of Chicago Pediatric Infectious Diseases.
At Lurie, Julia Payne, age 2, has been fighting AFM for almost a month.
"Nobody really knows if she will make a full recovery or this is, will be long-term," said her father, Jos Payne.
She was released into rehabilitation treatment Wednesday afternoon. Her parents said she is doing better.
Before being diagnosed, Julia's parents noticed she was having trouble holding up her head and arm.
"This is such a rare, scary condition that we don't know much about, so as much awareness as we can raise, we're all about that," said Julia's mom, Katy Payne.
"We think that acute flaccid myelitis is an infection of the nerve cells," Dr. Bartlett said. "It causes them to become inflamed and not work as well and become swollen."
Julia's symptoms are typical of AFM, but can also include difficulty swallowing, slurred speech and facial droop. The virus is transmitted via coughing, sneezing and not properly washed hands.
"AMF itself is a syndrome, is not something we believe is spread from person to person. It's probably more likely to be the after effects of having had an infection of respiratory disease," said Dr. Nirav D. Shah, director of the Illinois Department of Public Health.
There is no treatment for AFM so doctors treat the symptoms and provide physical therapy, but the long-term effects are not truly known.
The illness predominantly affects children and young adults.
"Since this potentially can be a polio virus, or a polio-like virus, you want to make sure you're up-to-date on your polio vaccine, but mainly handwashing and being on the look-out for symptoms," said Scott Goldstein, a pediatrician at Northwestern.
"Really the only thing we can do to protect ourselves is the same thing we do year-round to try to prevent infections," said Bartlett.