However, no new cases of the respiratory disease, caused by the bacteria legionella, have been reported.
A patient admitted with pneumonia at Harford Memorial Hospital on June 28 and diagnosed with Legionnaires' on July 2, died Thursday afternoon, the hospital said.
"This patient was one of the five original patients," hospital spokesman Bob Netherland said Thursday of the latest death.
Four other people who were patients at the hospital between May 1 and July 3rd were diagnosed with the disease. Two died in June and a third died on July 9.
Legionnaires' is a form of pneumonia caused by a common bacteria often found in air conditioning cooling towers, hot water tanks, whirlpool spas and humidifiers. The elderly, smokers and people with respiratory problems are at greatest risk of contracting the disease.
Despite the latest death, hospital officials think the spread of the disease has been contained.
The hospital's water supply was flushed with 150-degree water on July 3 to eradicate any lingering bacteria. Tests on the water system traced the bacteria to one of the hospital's hot water tanks.
The hospital began a continuous chlorination of the water system in accordance with the recommendations of the state health department, the Center for Disease Control in Atlanta and a water treatment consultant.
Netherland said there have been no new diagnoses since the hospital flushed its water supply.
Wednesday was the end of the incubation period for any bacteria that may have survived the hot water treatment. Any cases after Wednesday would indicate that the tank was not the only bacteria source.
Nurses from Harford Memorial contacted 300 of the 400 patients who were in the hospital from May 1 to July 3 to inform them of the potential risk. A hospital hotline to answer questions about Legionnaires' has received close to 700 calls.
Meanwhile, the legionella bacteria was detected in the cooling system of a building at the Naval Academy in Annapolis during a routine test. The Naval Academy closed its engineering building, Rickover Hall, after discovering the bacteria in the air conditioning cooling tower.
Naval Academy spokesman Cmdr. Mike Brady said the bacteria was in an isolated area that could not contaminate the water or air systems that cadets and other academy personnel use.
The building was closed Wednesday and will remain closed until at least Monday. More test results were expected Friday.