Enteroviruses are very common, causing between 10 and 15 million infections a year; many people who are infected with such a virus don’t get sick, or only develop a cold, the CDC says. But this uncommon strain, known as enterovirus 68, can cause severe breathing problems and has not appeared very often since it was first isolated in California in 1962.

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“Severe respiratory illness is always a concern to us, especially when children are affected,” Anne Schuchat, head of the CDC’s National Center for Immunizations and Respiratory Diseases, said Monday.

The suspected cases are scattered in clusters around the country, and authorities have said they are ultimately not sure how many infections this strain could cause or how widely it could spread. Only children have been confirmed to have been infected so far, as adults are unlikely to get diagnosed with this type of virus, Schuchat said during a conference call with reporters. (Children lack the immunity that comes from being exposed to a disease.)

“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.

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No vaccination is available for this strain.

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The CDC said Monday that a dozen states had suspected cases of the strain. Half of those states had confirmed cases of the rare enterovirus, according to the CDC and state health officials: Missouri, Illinois, Iowa, Kentucky, Colorado and Kansas. In addition, there are suspected cases in Alabama, Georgia, Michigan, Oklahoma, Ohio and Utah, the CDC said Monday.

For states with suspected clusters but no confirmation, and for public health officials wondering about the breadth of the situation, they must now wait for the CDC’s test results. (While hospitals are able to test for enteroviruses, most are not able to check which particular strain they are facing, so states are sending samples from children to the CDC.)

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Utah has no confirmed cases yet, but authorities suspect that the virus is likely there, said Tom Hudachko, a spokesman for the Utah Department of Health. Primary Children’s Hospital in Salt Lake City said it has seen an increase in children admitted with symptoms of serious respiratory diseases. Samples from this hospital will be sent to the CDC for testing, Hudachko said.

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There are also no confirmed cases in Ohio, but health officials are working with children’s hospitals in Cleveland and Cincinnati to send samples to the CDC. Similarly, authorities expect that some of the test could come back positive, said Melanie Amato, a Health Department spokeswoman.

The Michigan Department of Community Health reported an increase in children with respiratory illnesses across the state and is sending samples for testing. In Alabama, the state Department of Public Health has received multiple reports of suspected cases of this strain, but nobody has tested positive for it yet. The state is looking into two clusters of respiratory illnesses — one centered in Mobile, the other in the northern part of the state — and half a dozen samples have been sent for testing. They don’t expect test results for between one and two weeks.

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Officials in other states said they had already heard back from the CDC that they had children stricken with this strain. Children’s Hospital Colorado sent 25 samples to the CDC, and 19 of them tested positive, according to the state Department of Public Health and Environment. Specimens sent to the CDC from central Iowa tested positive, and reports of respiratory illnesses elsewhere in the state suggested “some activity statewide,” said Ann Garvey, the deputy state epidemiologist.

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There were 11 positive tests from 14 samples sent by a Chicago hospital, a spokeswoman for the Illinois Department of Public Health said. And there are more reports of respiratory illnesses around the state, she said. Kentucky sent 10 samples to the CDC after a surge in respiratory illnesses in the central part of the state and half of them tested positive, the state’s Cabinet for Health and Family Services said.

Last month, 141 children from Kansas were treated at Children’s Mercy Hospital in Kansas City, Mo. Some of these children were found to have the rare strain, said a spokeswoman for the Kansas Department of Health and Environment.

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Children’s Mercy saw nearly 500 children for respiratory problems, said Mary Anne Jackson, director of the hospital’s infectious diseases division. These children ranged in age from six weeks to old to 16 years.

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The CDC said that 19 of the 22 samples sent from Children’s Mercy tested positive for the rare virus strain. But Jackson said that she thinks between 70 and 90 percent of the children they are treating will be confirmed for this strain, which would mean hundreds of infections.