A third person has died following an outbreak of respiratory illness at a Fairfax County assisted-living facility that began more than two weeks ago, county health officials said Wednesday.

The outbreak at Greenspring Village in Springfield also spread to the unit’s staff, affecting 19 employees, Fairfax County Health Department officials said.

At a news conference Wednesday at the agency’s headquarters, Benjamin Schwartz, director of epidemiology and population health at the Fairfax County Health Department, said tests, including those conducted on 17 samples by the federal Centers for Disease Control and Prevention, have failed to identify a likely cause. Tests for Legionnaires’ disease have also come up negative. Officials tested for a range of common virus- or bacteria-borne respiratory illnesses.