Daniel Ramirez
Daniel RamirezFacebook

A six-year-old boy from Washington died after falling ill from a mysterious polio-like illness that has affected seven other children in the US state and dozens more across the country, CBS News reported. Daniel Ramirez was rushed to Seattle Children's Hospital last month with an unknown virus that caused his brain to swell.

According to a GoFundMe page set up by his family, he was put into a medically induced coma, which doctors hoped would help his brain heal. But on Monday morning, his family announced that Daniel lost his battle to the virus.

"It saddens us to announce that Daniel passed away on Sunday, October 30, 2016, surrounded by his family," they posted on Facebook.

"Daniel was an amazingly sweet little boy, who could put a smile on anyone's face. He had a personality that made him loved by everyone who ever met him. Daniel was taken from us too soon, but his memory will live on, and he will never be forgotten. Rest In Peace Sweet Daniel!" the post added.

CBS Seattle affiliate KIRO reported state and federal health officials were investigating it as a possible case of a rare condition that causes paralysis, called acute flaccid myelitis (AFM) – which is on the rise in American children.

According to the CDC, 89 people in 33 states have been confirmed to have AFM so far this year, most of them children. That's up from 21 cases in 2015.

There was also a surge of cases in the second half of 2014; the CDC says it received reports of 120 cases across the US between August and December that year. Experts say they're worried about why the number of cases seems to be going up again.

"The CDC is concerned about the increase in cases, so we're actively investigating the cases and working really closely with health departments on it. We're intensifying our efforts to find out what causes it – we don't know what causes it," CDC paediatrician Dr Manisha Patel told CBS News at the beginning of October.

AFM can affect anyone at any age. It affects a person's nervous system – specifically the spinal cord – and can result from a variety of causes, including viral infections.

Though there has been an increase in cases, and it is unknown what is behind it, the CDC emphasizes that AFM is still very rare.