Connecticut and New York are the two latest states to report possible cases of Enterovirus D68, the respiratory illness suspected of hospitalizing hundreds of children in 21 states in the South, Midwest and now Northeast.

New York became the first state on the East Coast to report cases of the virus that has been attacking children, especially those with asthma. On Friday, the New York State Department of Health announced the virus has been confirmed in more than a dozen children in the capital region and central New York. Onondaga County Health Department’s Medical Director Quoc Nguyen said two patients were in Onondaga County.

Although Enterovirus D68 has caused concern, particularly among parents of young children, enteroviruses themselves are common. The CDC estimated that 10 to 15 million people are infected with them each year. The Enterovirus D68 strain, in particular, dates to 1962 and hasn’t been widely reported in years, which experts said could be part of the problem.

“What’s unusual this time is this strain is causing severe respiratory disease,” Kennedy University Hospital Chief of Infectious Diseases David Condoluci told NJ.com. “We haven’t seen this strain for a long time, so a lot of kids don’t have the immunity for it.” In addition, there are no available vaccines or specific treatments for the virus, according to the CDC.

Mild symptoms typically look like those of a common cold: fever, runny nose, couching and sneezing, body and muscle aches. Many who became severely ill had asthma and experienced difficulty breathing and wheezing.

The Centers for Disease Control and Prevention sent out a health advisory stating it was working with hospitals and health departments to investigate such cases. The CDC has not yet confirmed the cases in New York State.

On Saturday, the Connecticut Department of Public Health said in a statement two hospitals in different parts of the state reported suspected cases of Enterovirus D68. The state health department said it is working with those hospitals and the CDC to facilitate laboratory testing.

Nicholas Bennett, head of the infectious disease department at Connecticut Children’s Medical Center, told FOXCT last week he saw a handful of patients whom he “highly suspects” have the fast-spreading virus.

“We’re starting to see a few just in the last day or so, where I’m really suspicious that’s what’s happening,” he told the station. He said at least one was treated in the Intensive Care Unit at the medical center.

After a surge in cases of severe respiratory illnesses last month popped up in Illinois and Missouri, the CDC ran tests and determined dozens of the children who were hospitalized there tested positive for a rare enterovirus strain. Most recently, Alabama and Washington state hospitals have seen an influx of sick children. Georgia, Michigan, Ohio, Oklahoma and Utah are also now investigating cases of respiratory illnesses, CNN reported.

The CDC since confirmed nearly 100 cases in six states: Colorado, Illinois, Iowa, Kansas, Kentucky and Missouri. Some state laboratories may have confirmed cases too, but those were not included in the CDC’s total.

According to the CDC, Enterovirus D68 is spread by coughing, sneezing or touching contaminated surfaces. Although the virus could potentially infect adults, most cases have been reported in children.

“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’s State Health Commissioner 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.”

Related: What is Enterovirus 68?