By Dennis Thompson

MONDAY, Sept. 8, 2014 (HealthDay News) -- A respiratory virus has stricken more than 1,000 children across 12 states, causing many to wind up in the hospital and prompting concerns of a wider outbreak.

About 15 percent of more than 300 children treated for respiratory illness in Missouri have ended up in an intensive care unit, according to a health alert from the Missouri Department of Health and Senior Services.

Children's Hospital Colorado reports that 86 kids have been hospitalized out of more than 900 treated for severe respiratory illness since Aug. 18.

The number of hospitalizations reported so far could be "just the tip of the iceberg in terms of severe cases," Mark Pallansch, director of the U.S. Centers for Disease Control and Prevention's Division of Viral Diseases, told CNN.

Public health officials fear the beginning of the new school year could allow the virus to gain momentum. But illness associated with the infection typically lasts about a week with cold-like symptoms, and all children have recovered completely, health officials said.

The CDC has identified the culprit as a rare respiratory virus known as Enterovirus D68, or EV-D68, that's part of the family of viruses that includes the common cold.

EV-D68 is largely responsible for two clusters of respiratory illness in Kansas City, Mo., and Chicago, Assistant Surgeon General Dr. Anne Schuchat, director of the U.S. National Center for Immunization and Respiratory Diseases, said during a Monday news conference.

A CDC lab found that 19 of 22 specimens from Kansas City and 11 of 14 samples from Chicago tested positive for EV-D68, Schuchat said. The virus has targeted children, infecting kids from 6 weeks to 16 years old, she said.

The CDC is asking doctors and public health officials to consider EV-D68 a potential suspect if widespread respiratory illnesses start occurring in their communities, she said.

"We believe that the unusual increases in Kansas City and Chicago may be occurring elsewhere over the weeks ahead, so we want people to be on the lookout," Schuchat said.

Schuchat was reluctant to estimate how many children nationwide have come down with EV-D68, given that many other respiratory illnesses are also making the rounds.

"We're in a stage where it's difficult to say just how big this is, how long it will go on for, and how widespread it will be," she said.

Children infected with this virus will appear to have a severe cold, with runny nose, sneezing and cough, according to Children's Hospital Colorado. But the illness can escalate quickly in some cases, and the child may start to have trouble breathing.

Missouri, Colorado, Georgia, Illinois, Iowa, Kansas, Kentucky, North Carolina, Ohio and Oklahoma have sent samples to the CDC for analysis, according to news reports.