A Fairfax County, Va., senior citizens residence has closed its assisted-living unit to new admissions and taken additional steps to halt an outbreak of respiratory illnesses after the death of two residents, county health department officials said Friday.

The move comes after 55 people became ill, 20 of whom required hospitalization. Two of those hospitalized died after suffering respiratory infections from an unknown pathogen at Greenspring, a retirement community in Springfield.

Benjamin Schwartz, director of epidemiology and population health in Fairfax County’s Health Department, said tests on those infected have come back negative for common virus- or bacteria-borne respiratory illnesses, including the flu and Legionnaire’s disease.

Samples have been sent to the Virginia Health Department in Richmond and forwarded to the federal Centers for Disease Control and Prevention in Atlanta for additional testing.

“We don’t know the cause yet,” Schwartz said. He said there are so many potential causes of respiratory illness that it’s possible a cause won’t be determined. It’s also difficult to know whether the illness played a role in the deaths of the two patients because both had preexisting medical conditions, he said.

In the meantime, patients who have become sick are being quarantined in their rooms. Some group activities have been canceled, and additional inspection, cleaning and sanitary precautions have been put in place, Schwartz said.

The complex is operated by Erickson Living, which serves about 24,000 people in a network of campuses for older people in 11 states. Courtney Benhoff, an Erickson Living spokeswoman, said the Springfield location’s staff was working closely with the county Health Department to halt the outbreak. She also said about half of those residents who were hospitalized have since returned to the complex, and some were diagnosed with conditions that were unrelated to the outbreak of respiratory infections.

The illnesses have affected only residents in Greenspring’s continuing-care and assisted-living unit, which houses 263 people. The symptoms ranged from coughing to pneumonia that required hospitalization, Schwartz said.

The outbreak began when a person became sick on June 30, and county health officials were alerted Monday, Schwartz said. He said Greenspring has done an “excellent job” keeping the county informed of developments and following the Health Department’s recommendations to stop the spread of infection.