Twenty-nine states and the District of Columbia now have a total of 213 confirmed cases of Enterovirus D68, the severe respiratory illness that typically targets children. So far, all the cases have involved children, except for one adult, according to the federal Centers for Disease Control and Prevention.

Officials said the 29 states are Alabama, Arkansas, California, Colorado, Connecticut, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, New Jersey, New York, North Carolina, North Dakota, Oklahoma, Pennsylvania, Virginia, Utah, Washington and West Virginia.

Enterovirus D68 is part of the family of viruses that includes the common cold. It can sometimes require hospitalization, especially for children with asthma.

More cases will be confirmed in the coming weeks because the testing for Enterovirus D68 is complex and can only be done by the CDC and a small number of state-run laboratories, the CDC said.

The first cases were diagnosed last month in Midwestern and Western states.

Enteroviruses are common in September as kids head back to school, with an estimated 10 million to 15 million people infected each year. But doctors believe this particular type of enterovirus is causing more severe cases than in the past.

The virus is also hard to track because it causes symptoms similar to many other infections, including the common cold, according to health experts.

“It is important that we follow common sense rules to prevent the spread of this virus, as we do for flu and other contagious illnesses,” New York state acting Health Commissioner Dr. Howard Zucker said in a statement. “Because there is no specific treatment or vaccination against this virus, our best defense is to prevent it by practicing proper hygiene.”

Antibiotics won’t work against a virus, and there is no antiviral treatment available for Enterovirus D68, health officials said.