Scientists trace Legionnaires' disease origins to 1976 outbreak and note rising cases

More than 200 attendees of the American Legion Convention at Philadelphia's Bellevue-Stratford Hotel fell ill with pneumonia in late July 1976, and 34 died. The cause was unknown until CDC microbiologist Dr. Joseph McDade isolated a previously unidentified bacterium, later named Legionella pneumophila, which multiplies in water systems rather than spreading person-to-person.

Legionella thrives in biofilms that develop on wet surfaces such as air-conditioning cooling systems, plumbing, whirlpools, humidifiers and decorative fountains. When aerosolized, the bacteria can be inhaled, leading to illness ranging from mild respiratory symptoms to severe pneumonia. Modern guidelines advise maintaining hot-water temperatures above critical levels and preventing stagnation to limit bacterial growth.

Initial treatment relied on erythromycin, but current protocols favor newer antibiotics such as azithromycin or levofloxacin. Rapid urine tests now enable quicker diagnosis, and DNA-based methods are expanding the ability to detect diverse Legionella species.

Outbreaks have continued worldwide, including a 1999 spa incident in the Netherlands, a 2015 cooling-tower outbreak in New York’s South Bronx, and a 2025 event in Harlem that caused 90 hospitalizations. In the United States, confirmed cases have risen to about 2.5 per 100,000 people annually—a fivefold increase since 2000—resulting in an estimated economic burden exceeding $1 billion per year. Seasonal peaks occur during warm, humid conditions, prompting routine monitoring in hospitals, hotels, cruise ships and office buildings.

"Maintaining hot-water lines above critical temperatures and avoiding stagnant zones are essential to reduce Legionella risk," according to CDC guidelines.

Researchers continue to develop risk-assessment tools for building water systems, aiming to curb future outbreaks.