(CNN) There are now 286 cases of possible and confirmed acute flaccid myelitis in the United States this year, according to the US Centers for Disease Control and Prevention.

That includes 116 confirmed cases of the rare polio-like illness, also known as AFM, that leads to sudden onset of paralysis. That's 10 more confirmed cases than the agency reported a week ago. An additional 170 possible cases of AFM are under investigation.

Since 2014 there have been 440 confirmed cases of AFM, most of them children. More than 95% of the patients with AFM this year have been children younger than age 18. The average age of those infected is 5, according to the CDC. Most of the children with confirmed cases experienced a viral illness with symptoms including fever and cough about three to 10 days before the onset of paralysis, the CDC said this month

The patients with confirmed AFM are in 31 states, which the CDC has now identified for the first time. There are 15 cases in Colorado, the state with the highest number, followed by Texas with 14 confirmed cases. Nineteen states have no confirmed cases, and 12 states are reporting only one confirmed case, according to the CDC.

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It's not clear whether there is more of a risk of AFM in states that have a higher number of cases or if those states are just better at identifying and reporting patients. Although the CDC has encouraged doctors to report cases, there is no requirement to do so.

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