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When members of the Pennsylvania American Legion gathered in Philadelphia on July 21, 1976, they were unaware that their organization’s annual conference would enter the epidemiological history books.
America was in a celebratory mood that month. The country had just marked the bicentennial of the nation’s founding, and Philadelphia, as a leading player in the revolutionary drama, was a center of revelry. Queen Elizabeth II had just visited.
Against that festive backdrop the Pennsylvania legion chapter convened in late July at the Bellevue Stratford Hotel, a grand French Renaissance building on Broad Street in the city center, for four days of meetings. Almost immediately, attendees started to fall ill with a mysterious respiratory infection.
By the end of the outbreak, some 182 people were sickened and 29 died. Doctors initially were befuddled. What was it? A poison? A virus? Where did it come from?
A six-month investigation – the “most extensive epidemiological investigation in U.S. history,” the Philadelphia Inquirer asserted– by the Center for Disease Control and the Pennsylvania Department of Health identified the culprit. What had attacked the lungs of conventioneers was not a virus, but a bacterium. It was christened Legionella pneumophila, the lung lover.
In the half-century since that first documented outbreak, researchers, doctors, and environmental engineers learned much more about the causes of Legionnaires’ disease, which is now the deadliest waterborne illness in the United States and one of the most fiendishly difficult to control. Case rates have increased more than five-fold since 2000. Large, deadly outbreaks still occur. One is ongoing today in New York City, where 74 people have been sickened on the Upper East Side and three people have died.
“We now know a lot more about the environmental niches in which Legionella proliferates,” said Charles Haas, a professor of environmental engineering at Drexel University. “We understand the levels that pose a significant risk to susceptible populations. And I think we’re developing a better understanding of how to appropriately control for those risks.”
The bad news, Haas said, is that Legionella is a formidable foe, widespread and continuing to wreak havoc. Knowing how to control the bacteria is one thing; but doing so is another.
In July 2025, another outbreak in New York City sickened 118 people, hospitalizing 92 and killing seven. In a September 2025 outbreak in Marshall County, Iowa, 74 cases were reported and two people died. The risk is not limited to the U.S. An outbreak in May in Hamilton, Ontario, hospitalized 22.
The continuation of these outbreaks is due to the nature of the illness. Legionnaires’ disease resembles pneumonia and is contracted by inhaling water droplets that carry the namesake bacteria. It is not spread person-to-person. The bacteria are found in rivers and lakes, but they multiply in warm, stagnant water inside building plumbing systems, especially on the slimy biofilms that form inside pipes. Legionnaires’ is a disease of the built environment, and older people, smokers, and those with weakened immune systems are most susceptible.
Within buildings the bacteria find any number of nooks and crannies in which to grow. To trigger an outbreak, all they need is a method of dispersal. Any mist will do – a rooftop cooling tower, a shower, a hot tub, a decorative fountain. The more public the dispersal source, the higher risk of a large outbreak. Rooftop cooling towers were the origin of the New York City incident last year, and they infected people who lived more than a half mile away.
Though the disease is a well-recognized and exceptionally serious threat, the federal government has been slow to respond. In fact, regulatory gaps provide Legionella a window of opportunity. For drinking water contaminants, the EPA generally does not regulate beyond the water meter, said Alan Roberson, former executive director of the Association of State Drinking Water Administrators. In other words, federal authority stops at the property line.
In a rules revision due next year, the EPA might finally develop stronger measures. The agency could issue a regulation to require water utilities to add sufficient levels of chlorine to disinfect water flowing throughout their distribution system. Even if the standard is finalized, however, building owners will remain responsible for water quality inside their structures. To Roberson, that continues to leave a gap.
“You shift the burden to the water utilities and if there’s still nothing done in buildings, you’re not solving the problem,” Roberson said.
Building-owner requirements are a patchwork. New York City, because of frequent Legionnaires’ outbreaks, has stringent registration and testing requirements for cooling towers. And hospitals, another high-risk location, generally have water management plans.
But, Haas said, the difficulty with regulating building owners is implementation. Think of all the hotels, hospitals, office towers, and apartment complexes spread across the country.
“A regulatory approach is really going to be incomplete,” Haas said. “There needs to be strong education.”
Public attention waxes and wanes for Legionnaires’ disease. Big outbreaks – the one in New York City last year or the one at McLaren Flint Hospital in 2014-2015 that was linked to the Michigan city’s disastrous lead contamination crisis – burn bright locally and then tend to fade.
That was not the case in 1976. The landmark contagion prompted a congressional hearing in November of that year. A half century later, the mysterious disease is less mysterious but still potent. For Haas and others, the next phase of disease prevention starts with a culture shift.
“To mitigate Legionella risk,” Haas said, “really requires diligence on the part of everybody who operates a cooling tower, everybody who operates some device to spray water anywhere, individual homeowners with their showers and their sprinkler systems, you name it.”
