The Wanaque Center for Nursing and Rehabilitation, located in Haskell, has been instructed by the state's health department not to admit any new patients until the outbreak has ended.
There have been 18 cases of adenovirus at the Wanaque Center for Nursing and Rehabilitation in Haskell, about 30 miles (50 kilometers) northwest of NY, the New Jersey Health Department said in a statement.
The virus is common in young children and usually causes mild symptoms.
"The combination of a worse strain of adenovirus together with a fragile population has led to a more severe outbreak", the statement added.
"This strain has been particularly associated with disease in communal living facilities", the department said in a statement.
A team from the Department of Health is investigating the possible causes of the outbreak.
New Jersey Governor Phil Murphy said in a statement Tuesday that state and local health officials are working to contain the outbreak.
The facility houses an adult nursing home, a rehabilitation center and a pediatrics unit.
The Health Department didn't release the ages of the victims or address the severity of the illness in the other dozen cases.
A health department team is at the facility, and an inspection team was also there Sunday. Unrelated Medicare investigations in 2017 and 2016 found "infection control" issues at Wanaque, including failing to properly clean and disinfect surfaces and objects inside the facility, improper storage of syringes and other tools and inadequate hand-washing practices.
The facility's paediatrics centre serves newborns to 22-year-olds who are "medically fragile" and provides an individualized program for each child, the centre's website said.
Local news site NewJersey.com reported that a letter had been sent to parents of patients about an outbreak on 18 Oct, but officials did not confirm any details about the virus until Tuesday.