Legionnaires' disease, which has killed two people at the Bethesda naval hospital during the past month, was confirmed in a third patient Tuesday evening, a hospital official said yesterday.

The unidentified man is undergoing treatment for the rare form of pneumonia, but hospital spokesman Lt. Cmdr. Claude Shehane would not reveal the patient's condition.

Shehane said the latest patient was staying on an inpatient ward in Building 10 where the bacterium legionella, believed to cause the disease, had been discovered. "But there were not positive cultures from the room that he is in," Shehane said when explaining that earlier tests had found no evidence of the bacteria in that room.

"Of course we're concerned," Shehane said of the latest case, but added that "this is still not an epidemic."

Experts say Legionnaires' disease no longer inspires the terror it did when it was discovered eight years ago with the deaths of 29 people at an American Legion convention in Philadelphia. Since then, antibiotic treatment has greatly reduced the death rate from the disease, which strikes an estimated 25,000 people each year.

However, the disease still kills about 20 percent of its victims who are already sick or are undergoing treatment that weakens their immunity systems.

Hospital officials, who would not reveal the nature of the illness that had originally hospitalized the latest Legionnaires' disease patient, said the first two Legionnaires' disease patients at the hospital, a man and a woman, were both being treated for terminal cancer.

Testing at the hospital revealed colonies of the bacterium legionella on shower heads and faucets in 17 of 91 places around the 500-bed hospital and surrounding buildings, Shehane said.

The contaminated shower heads and faucets were changed, Shehane said, and the areas treated. Shehane said no more contaminated sites have been found, but retesting continues on the previously infected areas.

Dr. Margaret Oxtoby, an epidemiologist at the Centers for Disease Control in Atlanta, said Legionnaires' disease is rare and easily treated in healthy people, but fairly common -- and serious -- in hospitalized patients whose immunity systems have been weakened.

Oxtoby said that while she is not aware of any current cases of Legionnaires' disease in other Washington area hospitals, "it's expected to find a certain number of cases" in any hospital that treats patients with severe illnesses.

The number of clusters of the bacteria found at the Bethesda hospital was not unusually high, she said.