Most of those who died in the adenovirus outbreak were under 18, but at least one was a young adult, the state Health Department said. The seventh victim died Tuesday.

There have been 18 cases overall at the Wanaque Center for Nursing and Rehabilitation in Haskell, about 30 miles (50 kilometers) northwest of New York, officials said. The 227-bed, for-profit facility has a pediatric unit but also cares for elderly residents.

All the cases occurred in a respiratory, or ventilator unit, New Jersey Health Commissioner Dr. Shereef Elnahal said at a news conference with Gov. Phil Murphy outside the complex.

“Our attention is squarely focused on making sure that further cases are minimized,” Murphy, a Democrat, said.

Health officials said they have staff onsite, and that the outbreak won’t be declared over until the center goes four weeks without the diagnosing of a new case.

The commissioner said the first symptoms showed up Sept. 26, and that the state was notified of an outbreak Oct. 9. Asked about the time lag, he said it’s always hard to know when an actual outbreak begins.

“It can be difficult or impossible to know how the virus got to the facility, what its source was, or what its specific mechanisms of spread is from person to person,” Elnahal said. “The consequences here have been drastic, and this has been absolutely wrenching for families, community members, our department, myself personally and the governor.”

Adenoviruses are common and typically cause mild cold or flu symptoms in otherwise healthy people, including sore throats, fever, coughs and sneezes. Some strains also cause diarrhea and or pinkeye.

But the strain found in the rehab center outbreak, adenovirus 7, is among the more potent types and can sometimes cause more serious respiratory illness, particularly in people with weak immune systems or who have lung conditions.

Many of the children at the facility require ventilators for breathing, and some have spent all or most of their lives there, the commissioner said. He described the patients in an interview as “fragile.” It’s unlikely that there’s a broader threat, he said.

“Unfortunately, we have hundreds of these types of outbreaks per year,” the commissioner said in the interview. “Because this one occurred in a place where the patients are so fragile and have underlying immune system problems, you’re seeing the severity play out.”

Elnahal said a surprise inspection at the facility this past weekend found only a handwashing deficiency.

The president of a nurses union said Wednesday that nurses have complained about staffing shortages at Wanaque, which she said may lead to poor infection control practices that could put patient safety at risk.

“As the staff works to contain the spread of the virus they will also continue to advocate for stronger protocols and implementation of infection control policies,” the president, Debbie White, said in a statement.

The center referred questions to a spokesman, who said late Wednesday that he was preparing a statement.

The identities of those who died and the affected patients have not been disclosed.

Health officials had earlier described all those infected as children, which is how the rehab center describes its pediatric patients, even though its website says some are as old as 22.

In the past 10 years, cases of severe illness and death from the type of infection found at Wanaque have been reported in the United States, but it’s unclear how many deaths there were.

A 1998 outbreak of type 7 adenovirus at a pediatric chronic-care facility in Chicago killed eight patients, according to a scientific paper cited by the Centers for Disease Control and Prevention.

The CDC is providing technical assistance to the state Health Department.

The Health Department inspected Wanaque in August and found “minor deficiencies” that were later corrected.

“Our goal now,” Elnahal said, “is to make sure all the infection protocols are being followed to minimize the impact on affected families and patients.”

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Associated Press writers Lindsey Tanner in Asheville, North Carolina, and Maryclaire Dale in Philadelphia contributed to this report.

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This story has been updated to correct that not all the outbreak victims were children. Officials had earlier described them all as children but now say that at least one was a young adult, and that the center describes all its pediatric patients as children, regardless of age.