The respiratory illness Enterovirus (EV-D68) is spreading and has now been confirmed in 277 patients in 40 states and the District of Columbia, the Center for Disease Control and Prevention (CDC) said on Friday.
Those numbers compare to 220 cases in 31 states reported by the CDC on Wednesday.
State health departments have reported an increase in severe respiratory disease in children this year.
"This increase could be caused by many different viruses that are common during this time of year," the CDC said. "EV-D68 appears to be the predominant type of enterovirus this year and may be contributing to the increases in severe respiratory illnesses."
The New Hampshire Department of Health and Human Services also confirmed its first two EV-D68 cases in two children from Hillsborough County, the agency said.
The EV-D68 virus, which was not common in the United States until recently, causes cold-like symptoms and can lead to serious breathing difficulties, especially in children who have asthma. Infants, children and teenagers are most at risk for EV-D68.
The CDC first spoke of the outbreak on Sept. 8 after investigating numerous reports of hospitalized children in Chicago and Kansas City.