Two cases apparently of what is known as legionnaires' disease - the first in Virginia - have been reported by the State Health Department.

A 79-year-old Fairfax County man died last month apparently as a result of legionnaire's disease, the department said. The man, whose name was not released had been under treatment for a chronic form of leukemia, according to Dr. Charles F. Lovell Jr., assistant director of the department's epidemiology bureau.

The state's second victim of the pneumonia-like disorder was a Roanoke-area woman in her 40s who was hospitalized from about mid-September until mid-October. She has recovered, department officials said.

The death of the Fairfax County man brings to six the number of putative victims of legionnaire's disease in the Washington area.

Four cases have occurred in the District and one in Maryland.

The disease was first identified in the summer of 1976 when it hit 180 persons and killed 29 among those who attended a Philadelphia American Legion convention.The bacterium that causes it was identified in January and scientists found the same "bug" had caused a 1965 epidemic at Washington's St. Elizabeths Hospital.

The tentative diagnosis of the two Virginia cases of legionnaires' disease were based on blood studies by the National Center for Disease Control in Atlanta. The State Health Department in investigating the Virginia cases.