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

Soon after Philadelphia hosted America’s bicentennial celebrations in late July 1976, more than 200 attendees of the American Legion Convention at the Bellevue-Stratford Hotel in Philadelphia fell ill with pneumonia symptoms, including fever, cough and trouble breathing.

Thirty-four of them died.

One of us was a newly minted pulmonary fellow at Hahnemann University Hospital. The hospital was just a mile from the hotel and received some of the first cases.

At the time, no one knew what caused the illnesses. Scientists considered bacterial or viral infection, heavy metal toxins or some combination of environmental causes.

Early days of Legionella pneumophila

Several months later, Dr. Joseph McDade, a microbiologist at the Centers for Disease Control and Prevention, discovered the organism that was responsible after doing detailed microbiological investigation and animal testing.

The newly recognized bacteria was named Legionella pneumophila after the outbreak at the Legionnaires’ convention.

Legionella pneumophila differed from infectious respiratory organisms known at the time since it multiplied in an environmental water source and was not spread by person-to-person contact.

It was also atypical because these bacteria do not have cell walls and appear colorless with the usual laboratory staining methods – making them difficult to identify.

The CDC found that this same organism had been responsible for earlier, milder clusters of respiratory illness in Pontiac, Michigan, that occurred in 1968. This milder disease form was named Pontiac fever.

Legionella can thrive in pipes, whirlpools and humidifiers

Legionnaires’ disease puzzled epidemiologists and microbiologists because it came from a type of exposure that had not received much attention.

The bacterium was found to multiply in biofilms – slimes that grow on wet surfaces. The air conditioner cooling systems at the Bellevue hotel in 1976 hosted such biofilms.

We now know that household pipes and plumbing fixtures, whirlpools, humidifiers and many other places can provide a habitat for Legionella pneumophila.

From these engineered systems, particles containing the bacteria can become airborne and be inhaled by nearby people. Disinfecting these systems can reduce growth of Legionella, while new designs can reduce the spread of aerosols.

Scientists now also know that Legionella pneumophila is just one of dozens of species of microorganisms that can cause respiratory illness from airborne water particles.

One of us, a professor of environmental engineering, has developed ways to assess the risk for many different environmental microorganisms. Students at Drexel University in Philadelphia used the methods to calculate critical concentrations of Legionella for water management.

Today, there are also consensus guidelines for managing building water systems, especially to reduce the risk of Legionnaires’ disease. For example, it is very important to keep buildings’ hot water lines above critical temperatures and to avoid stagnant zones in which chlorine in water can decay.

Evolving treatments for Legionnaires’

In the early days of treating Legionnaires’, the standard treatment became an antibiotic called erythromycin.

However, erythromycin had many side effects.

Today, doctors typically treat Legionnaires’ and other cases of severe community-acquired pneumonia with newer antibiotics like azithromycin or levofloxacin, which are commonly used for a variety of infections.

Legionnaire’s disease runs a spectrum from very mild disease or with moderate symptoms such as cough and chest congestion to severe cases that require prolonged hospitalization. Fortunately, rapid diagnostic tests have been developed that can detect the organism in urine. Hospitals use these tests to determine whether a patient with respiratory symptoms has Legionella.

Cases are rising and outbreaks continue

Much has been learned about the microbiology, clinical response, ecology and engineering factors linked to Legionella. Still, numerous outbreaks have occurred in the years since 1976.

Notable ones include contamination at a whirlpool spa at a flower show in the Netherlands in 1999 that caused at least 188 illnesses and 21 deaths. In 2015, a Legionnaires’ disease outbreak in multiple building cooling towers in the South Bronx in New York resulted in 138 cases and 16 deaths.

More recently, a Legionnaires’ outbreak that began in late July 2025 in the Harlem neighborhood of New York caused 90 hospitalizations and seven deaths.

Reported cases of Legionnaires’ disease in the U.S. have increased in recent decades. Currently there are about 2.5 confirmed cases per 100,000 people per year. According to the CDC, that’s a fivefold increase since 2000.

The CDC estimates that the U.S. economic burden from Legionnaires’ disease is likely more than US$1 billion per year.

Cases peak during warm weather, especially in humid conditions or after rain. Legionella bacteria can grow in cooling towers, hot water tanks, hospital plumbing systems and decorative fountains. Today, hospitals, hotels, cruise ships and office buildings all have routine Legionella monitoring.

Also, different species of Legionella can cause other respiratory illnesses besides Legionnaires’ disease. Current clinical tests may miss much of the diversity of Legionella, but molecular biological tools are improving quickly, and DNA-based methods are expanding the diagnostic tool kit.

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