Jennifer Cornejo of Lone Tree in Colorado told News7 in Denver her 13-year-old son William had cold symptoms that developed overnight into a life-threatening respiratory illness. “He was in really bad shape,” she said. “He came really close to death. He was unconscious at our house and white as a ghost with blue lips — he just passed out.”

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“My head started hurting,” William said. “And after that my lungs started closing up. It felt different.”

Hospitals in Colorado, Missouri and potentially eight other states are admitting hundreds of children for treatment of an uncommon but severe respiratory virus.

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The virus, called Enterovirus D68, causes similar symptoms to a summer cold or asthma: a runny nose, fever, coughing and difficulty breathing. But the illness can quickly escalate, and there are no vaccines or antiviral medications to prevent or treat it.

Though only Missouri has confirmed cases of EV-D68, cases with similar symptoms have been reported throughout the Midwest and South.

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According to news reports, Missouri, Colorado, Georgia, Illinois, Iowa, Kansas, Kentucky, North Carolina, Ohio and Oklahoma have sent samples to the Centers for Disease Control for identification. Hospitals usually aren’t able to perform testing required to identify specific types of enteroviruses on their own.

This particular outbreak is associated with an unusually high number of hospitalizations — Children’s Hospital Colorado has reported more than 900 cases since Aug. 18, while Children’s Mercy Hospital in Kansas City, Mo., has seen about 30 children per day with the illness, the Denver Post reported.

Mark Pallansch, director of the CDC’s Division of Viral Diseases, said that the recent hospitalizations may be “just the tip of the iceberg in terms of severe cases.”

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“We’re in the middle of looking into this,” he told CNN on Sunday. “We don’t have all the answers yet.”

Enteroviruses are common in the United States, causing 10 to 15 million infections per year, most of which are mild or asymptomatic. Infections usually occur in the summer and fall, coinciding with the start of the school year, and the viruses are most often found in infants, children and teenagers, who haven’t yet built up an immunity.

EV-D68, which is likely causing this most recent outbreak, almost exclusively affects the upper respiratory tract and is especially severe in people who already experience breathing difficulties.

The virus was first identified in 1962, but has been relatively rare until recently. Between 1987 and 2005, 26 cases were reported worldwide — in the past month, 19 have been confirmed from Kansas City alone.

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Identifying particular types of enteroviruses is difficult, but a 2011 CDC report on EV-D68 pinpointed clusters of the disease in the Philippines, Japan, the Netherlands and three U.S. states since 2008.

Of the 90 confirmed cases described in the report, three were fatal, none of which occurred in the United States. But an editorial note warns that the virus is “increasingly recognized” for causing severe — and potentially deadly — respiratory illness.

This year’s outbreak has hospitals scrambling to deal with the large number of cases.

“It’s worse in terms of scope of critically ill children who require intensive care. I would call it unprecedented,” Mary Anne Jackson, division director for infectious diseases at Kansas City’s Children’s Mercy Hospital, told CNN. She said the facility has had to call in help from other providers.

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Some hospitals, including Blessing Hospital in Quincy, Ill., are barring children — who are most vulnerable to the virus — from visiting as a result of the outbreak. Though it is still unclear whether EV-D68 is present at the facility, Blessing’s emergency center saw 70 pediatric patients with breathing difficulties since Labor Day weekend, seven of whom were admitted for treatment.

But early September is peak season for enteroviruses, and doctors say they expect the number of infections to begin leveling off.

“The vast majority of these kids will get better,” William Schaffner, chair of Vanderbilt University’s Department of Preventative Medicine, told NBC.