{"id":42670,"date":"2026-06-20T07:15:00","date_gmt":"2026-06-20T14:15:00","guid":{"rendered":"https:\/\/www.lifeandnews.com\/articles\/?p=42670"},"modified":"2026-06-20T07:52:18","modified_gmt":"2026-06-20T14:52:18","slug":"legionnaires-disease-outbreak-in-philadelphia-in-1976-was-mysterious-and-deadly-50-years-later-scientists-know-the-cause-but-outbreaks-continue","status":"publish","type":"post","link":"https:\/\/www.lifeandnews.com\/articles\/legionnaires-disease-outbreak-in-philadelphia-in-1976-was-mysterious-and-deadly-50-years-later-scientists-know-the-cause-but-outbreaks-continue\/","title":{"rendered":"Legionnaires\u2019 disease outbreak in Philadelphia in 1976 was mysterious and deadly \u2013 50 years later, scientists know the cause but outbreaks continue"},"content":{"rendered":"\n

Charles N. Haas<\/a>, Drexel University<\/a><\/em> and Robert Promisloff<\/a>, Drexel University<\/a><\/em><\/p>\n\n\n\n

\"Black-and-white
The landmark Bellevue-Stratford Hotel in Philadelphia went out of business after the Legionnaires\u2019 disease outbreak in 1976. Bettmann\/Bettman Collection via Getty Images<\/a><\/figcaption><\/figure>\n\n\n\n

Soon after Philadelphia hosted America\u2019s 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.<\/p>\n\n\n\n

Thirty-four of them died<\/a>.<\/p>\n\n\n\n

One of us<\/a> 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.<\/p>\n\n\n\n

At the time, no one knew what caused the illnesses. Scientists considered bacterial or viral infection<\/a>, heavy metal toxins or some combination of environmental causes.<\/p>\n\n\n\n

Early days of Legionella pneumophila<\/em><\/h2>\n\n\n\n

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

The newly recognized bacteria was named Legionella pneumophila<\/em> after the outbreak at the Legionnaires\u2019 convention.<\/p>\n\n\n\n

Legionella pneumophila<\/em> 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.<\/p>\n\n\n\n

It was also atypical because these bacteria do not have cell walls and appear colorless with the usual laboratory staining methods \u2013 making them difficult to identify.<\/p>\n\n\n\n

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<\/a>.<\/p>\n\n\n\n

\"Woman
Microbiologists had a hard time identifying the cause of the 1976 Legionnaires\u2019 outbreak, as it was an atypical bacterium. CDC\/Warren Jones, 1979\/Smith Collection\/Gado via Getty Images<\/a><\/figcaption><\/figure>\n\n\n\n

Legionella<\/em> can thrive in pipes, whirlpools and humidifiers<\/h2>\n\n\n\n

Legionnaires\u2019 disease puzzled epidemiologists and microbiologists because it came from a type of exposure that had not received much attention.<\/p>\n\n\n\n

The bacterium was found to multiply in biofilms \u2013 slimes that grow on wet surfaces. The air conditioner cooling systems at the Bellevue hotel in 1976 hosted such biofilms.<\/p>\n\n\n\n

We now know that household pipes and plumbing fixtures, whirlpools, humidifiers<\/a> and many other places can provide a habitat for Legionella pneumophila.<\/em><\/p>\n\n\n\n

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<\/em>, while new designs can reduce the spread of aerosols.<\/p>\n\n\n\n

Scientists now also know that Legionella pneumophila<\/em> is just one of dozens of species of microorganisms that can cause respiratory illness from airborne water particles.<\/p>\n\n\n\n

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

Today, there are also consensus guidelines for managing building water systems, especially to reduce the risk of Legionnaires\u2019 disease<\/a>. For example, it is very important to keep buildings\u2019 hot water lines above critical temperatures and to avoid stagnant zones in which chlorine in water can decay.<\/p>\n\n\n\n

\"Dozens
Legionella pneumophila<\/em> is a bacterium that causes Legionnaires\u2019 disease. Janice Haney Carr\/Centers for Disease Control and Prevention via AP<\/a><\/figcaption><\/figure>\n\n\n\n

Evolving treatments for Legionnaires\u2019<\/h2>\n\n\n\n

In the early days of treating Legionnaires\u2019, the standard treatment became an antibiotic called erythromycin.<\/p>\n\n\n\n

However, erythromycin had many side effects.<\/p>\n\n\n\n

Today, doctors typically treat Legionnaires\u2019 and other cases of severe community-acquired pneumonia<\/a> with newer antibiotics like azithromycin or levofloxacin, which are commonly used for a variety of infections.<\/p>\n\n\n\n

Legionnaire\u2019s 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<\/a>. Hospitals use these tests to determine whether a patient with respiratory symptoms has Legionella<\/em>.<\/p>\n\n\n\n

Cases are rising and outbreaks continue<\/h2>\n\n\n\n

Much has been learned about the microbiology, clinical response, ecology and engineering factors linked to Legionella.<\/em> Still, numerous outbreaks have occurred in the years since 1976.<\/p>\n\n\n\n

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<\/a>. In 2015, a Legionnaires\u2019 disease outbreak in multiple building cooling towers in the South Bronx in New York resulted in 138 cases and 16 deaths<\/a>.<\/p>\n\n\n\n

\"People
A rooftop air conditioning unit at the Opera House Hotel in the South Bronx in New York was the source of a deadly Legionnaires\u2019 outbreak in 2015. Spencer Platt via Getty Images<\/a><\/figcaption><\/figure>\n\n\n\n

More recently, a Legionnaires\u2019 outbreak that began in late July 2025 in the Harlem neighborhood of New York caused 90 hospitalizations and seven deaths<\/a>.<\/p>\n\n\n\n

Reported cases of Legionnaires\u2019 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\u2019s a fivefold increase since 2000<\/a>.<\/p>\n\n\n\n

The CDC estimates that the U.S. economic burden from Legionnaires\u2019 disease is likely more than US$1 billion per year<\/a>.<\/p>\n\n\n\n

Cases peak during warm weather, especially in humid conditions or after rain. Legionella<\/em> 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<\/em> monitoring.<\/p>\n\n\n\n

Also, different species of Legionella<\/em> can cause other respiratory illnesses besides Legionnaires\u2019 disease. Current clinical tests may miss much of the diversity of Legionella<\/em>, but molecular biological tools are improving quickly, and DNA-based methods are expanding the diagnostic tool kit.<\/p>\n\n\n\n

Read more of our stories about Philadelphia and Pennsylvania<\/a>, or sign up for our Philadelphia newsletter on Substack<\/a>.<\/em><\/p>\n\n\n\n

Charles N. Haas<\/a>, Professor of Environmental Engineering, Drexel University<\/a><\/em> and Robert Promisloff<\/a>, Clinical Professor Emeritus of Medicine, Drexel University<\/a><\/em><\/p>\n\n\n\n

This article is republished from The Conversation<\/a> under a Creative Commons license. Read the original article<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

Charles N. Haas, Drexel University and Robert Promisloff, Drexel University Soon after Philadelphia hosted America\u2019s 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 […]<\/p>\n","protected":false},"author":56,"featured_media":42672,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[42,827,10,118,3410,15533],"tags":[17758,2897,376,191,869,17860,885,891,886,860,14506,133,5260,5273,4222,8247],"_links":{"self":[{"href":"https:\/\/www.lifeandnews.com\/articles\/wp-json\/wp\/v2\/posts\/42670"}],"collection":[{"href":"https:\/\/www.lifeandnews.com\/articles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.lifeandnews.com\/articles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.lifeandnews.com\/articles\/wp-json\/wp\/v2\/users\/56"}],"replies":[{"embeddable":true,"href":"https:\/\/www.lifeandnews.com\/articles\/wp-json\/wp\/v2\/comments?post=42670"}],"version-history":[{"count":1,"href":"https:\/\/www.lifeandnews.com\/articles\/wp-json\/wp\/v2\/posts\/42670\/revisions"}],"predecessor-version":[{"id":42671,"href":"https:\/\/www.lifeandnews.com\/articles\/wp-json\/wp\/v2\/posts\/42670\/revisions\/42671"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.lifeandnews.com\/articles\/wp-json\/wp\/v2\/media\/42672"}],"wp:attachment":[{"href":"https:\/\/www.lifeandnews.com\/articles\/wp-json\/wp\/v2\/media?parent=42670"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.lifeandnews.com\/articles\/wp-json\/wp\/v2\/categories?post=42670"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.lifeandnews.com\/articles\/wp-json\/wp\/v2\/tags?post=42670"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}