Hundreds of children across America have been sickened in recent weeks by what health officials suspect is a rare respiratory virus.

The Centers for Disease Control and Prevention (CDC) believes that human enterovirus 68 is at the root of the epidemic, though testing of samples has not produced a definitive answer.

The Denver Post reported that officials at Children's Hospital Colorado have treated more than 900 children for severe respiratory illnesses since August 18, with 86 admitted to the hospital.

In Kansas City, Missouri, one hospital said 15 percent of the affected pediatric patients have been hospitalized.

Despite the enterovirus being around since the 1960s, CDC officials told Fox News' Dr. Marc Siegel they believe it has been "overlooked."

"We get 10 or 15 million cases of enterovirus per year," Siegel said on America's Newsroom on Monday. But what makes this virus different, is that it usually is a bowel disease, while this one is respiratory, Siegel explained.

"If a kid has asthma, they can get very, very sick, and the CDC believes that that may be why we're seeing a lot of hospitalizations," Siegel said.

At Rocky Mountain Hospital for Children, doctors put five children on ventilators this week, including a 13-year-old boy whose condition deteriorated from a mild cold to a life-threatening illness overnight.

"He was OK. Then he was unconscious. It was unreal," Jennifer Cornejo told the Post about her son William. "I thought my heart would come out of my chest. It was so horrible."

"You may not have immunity to it if you haven't seen it before and you're seeing it for the first time," Seigel said, adding that that may be why so many kids are getting sick.

The illness appears to be most common among very young children and children who have asthma. The CDC tells ABC News that similar suspected outbreaks have been reported in nine other states -- Missouri, Kansas, Illinois, Kentucky, Iowa, Ohio, Oklahoma, North Carolina, and Georgia. The Columbus Dispatch reported that Nationwide Children's Hospital in Ohio's capital reported seeing an average of 73 patients with respiratory complaints per day between August 31 and September 2. 

Doctors recommend taking basic sanitary precautions, including washing hands often with soap and water, avoiding sharing items with sick people, and disinfecting frequently touched surfaces. 

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