“What the C.D.C. is reporting is clearly the tip of the iceberg,” said Dr. Mary Anne Jackson, the division director of infectious diseases at Children’s Mercy Hospital in Kansas City, Mo. The hospital was the first to alert the agency last month to an unusual increase in children with trouble breathing. Since then, Dr. Jackson has received calls from colleagues nationwide seeking guidance. Some report that the influx of children to hospitals is “almost outweighing the resources available,” she said.

Three times in the past month, the University of Chicago Medicine Comer Children’s Hospital has had to divert ambulances to other hospitals because its emergency room was filled with children, most of them younger than 5, with severe respiratory illness. Before the outbreak, the hospital had not had to divert ambulances in 10 years, said Dr. Daniel Johnson, the interim section chief of pediatric infectious diseases at the hospital.

Enteroviruses are common, but this strain is not. Symptoms in the current outbreak resemble those of a bad cold, including body aches and cough. But some children progress to wheezing and having breathing difficulties. Scientists say they do not know why it is happening.