Legionnaires’ disease outbreak in Philadelphia in 1976 was mysterious and deadly - 50 years later, scientists know the cause but outbreaks continue

Key Points:

• In 1976, over 200 attendees of the American Legion Convention in Philadelphia fell ill with pneumonia-like symptoms, resulting in 34 deaths; this outbreak led to the discovery of Legionella pneumophila by CDC microbiologist Dr. Joseph McDade.
• Legionella pneumophila is unique as it multiplies in environmental water sources like biofilms in air conditioning systems, plumbing, and humidifiers, spreading through airborne water particles rather than person-to-person contact.
• Advances in water system management and building design, including maintaining hot water temperatures and preventing stagnant water, help reduce Legionella growth and transmission, while rapid diagnostic tests now aid in early detection of infections.
• Although treatments have evolved from erythromycin to newer antibiotics like azithromycin and levofloxacin, Legionnaires’ disease cases have increased fivefold in the U.S. since 2000, with outbreaks continuing to occur in various settings including hospitals and public buildings.
• The CDC estimates the economic burden of Legionnaires’ disease in the U.S. exceeds $1 billion annually, with cases peaking in warm, humid weather; ongoing improvements in molecular diagnostics are enhancing detection of diverse Legionella species beyond traditional tests.