Nearly 500 children have been treated at one hospital alone -- Children's Mercy in Kansas City, Missouri -- and some required intensive care, according to authorities.
The suspected germ, enterovirus 68, is an uncommon strain of a very common family of viruses that typically hit from summertime through autumn.
The virus can cause mild coldlike symptoms including runny noses, coughing and wheezing but Mark Pallansch, director of the viral diseases division at the federal Centres for Disease Control and Prevention, said this summer's cases are unusually severe and include serious breathing problems.
The virus typically causes illness lasting about a week and most children recover with no lasting problems.
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Authorities in Illinois and Colorado said their states are among those with suspected or confirmed cases. A CDC spokesman said the agency also is testing to see if the virus caused respiratory illnesses reported in children in Alabama, Georgia, Iowa, Kansas, Kentucky, Ohio, Oklahoma and Utah.
The CDC's Dr Anne Schuchat said at a news briefing yesterday that there are other viruses making kids sick.
Children with asthma and other health problems are especially at risk for the entervirus, but reported cases include children without asthma who have developed asthmalike breathing problems, Pallansch said. He said no deaths have been reported in the outbreak.
Dr Mary Anne Jackson, director of infectious diseases at Children's Mercy, said local cases began appearing in mid-August and they appear to have peaked in her area.
Schuchat, said the strain involved also appeared in the United States last year and in specimens from other countries.