Legionnaires’ Disease Hits the “Big Apple” Again

“Both the common [name Legionnaires’ disease] and official scientific name, Legionella pneumophila, convey the end result of a process, the hard essential work of public health. The very one that has been under attack during the second Trump administration.”

Jeremy Faust, MD, Editor-in-Chief, MedPage Today, July 13, 2026

“When public health works, there are no headlines.”

Source Unknown

Fifty years ago, members of the Pennsylvania State American Legion gathered in Philadelphia in July to celebrate this nation’s bicentennial. An outbreak of a severe respiratory illness, however, marred the celebration. Within a few weeks, 221 people were sickened with the disease and 34 died.

By January 1977, scientists at the Centers for Disease Control and Prevention (CDC) had isolated the bacterium that caused the disease, identified it as Legionella pneumophila—a pathogen that is now recognized as one of the most common causes of community acquired pneumonia in the developed world—and named the illness Legionnaires’ disease after the celebrants in Philadelphia that were stricken by it. This was one of the landmark discoveries of researchers at the CDC and happened at a time when this country saw the CDC as the leader in protecting this nation against emerging infectious diseases.

At present, the second outbreak of Legionnaires’ disease in just two years is occurring in New York City. I therefore decided to focus this week’s post on this serious bacterial infection.

What is Legionnaires’ disease (a recap)? Pneumonia (lung infection) is one of the leading causes of illness and death, killing about 2.2 million people globally each year.  Legionnaires’ disease is a severe type of pneumonia caused by L. pneumophila, a gram-negative bacterium with an affinity for fresh water. While more than 65 species of Legionella are recognized, L. pneumophila is the species that causes the vast majority of human cases of Legionnaires’ disease—between 8,000-18,000 hospitalizations per year in the U.S. alone, with a mortality of 10%.

Unlike most other waterborne pathogens, L. pneumophila is not acquired by drinking contaminated water. Instead, it is acquired by inhaling mist that contains the pathogen.

L. pneumophila thrives inside protozoa including free-living amoeba that form microbial biofilms on wet surfaces, including pipes of drinking water distribution systems, building plumbing systems, faucets, showerheads, hot tubs, fountains, and, especially, cooling towers. When the bacterium reaches the lungs, it seeks an intracellular environment, and alveolar macrophages, host cells that are akin to amoeba, are a perfect fit.

Symptoms and treatment. Common symptoms of Legionnaires’ disease are shared with many other flu-like illnesses and include cough, fever, shortness of breath, headaches, muscle pains, confusion, loss of appetite, nausea, and diarrhea. None of the symptoms, however, is specific for Legionnaires’ disease. Fortunately, there is a diagnostic test; the most commonly used is the urine antigen test which detects a molecule of L. pneumophila in the urine.

If diagnosed early, most cases of Legionnaires’ disease can be treated successfully with antibiotics. Selection of the antibiotic is guided by knowledge of the intracellular location of the bacterium. Therefore, antibiotics that penetrate alveolar macrophages, like macrolides and fluoroquinolones, are commonly used.

Adults aged 50 years and older, smokers, and those with chronic lung diseases or weakened immune systems are recognized as high risk groups for the disease.

Outbreaks. There is no official number of global outbreaks of Legionnaires’ disease per year. Hundreds of outbreaks that occur outside the U.S. do, however, make the news every year—especially when they occur in popular vacation venues. And the CDC records at least 20 outbreaks every year. The two back-to-back outbreaks in New York City are worth highlighting.

Central Harlem, summer 2025.  One year ago, an outbreak of Legionnaires’ disease occurred in Central Harlem resulting in 118 cases and 7 deaths. When the outbreak was first identified, New York City health officials directed an investigation into all cooling towers in the affected neighborhood. They sampled 43 cooling tower systems and found L. pneumophila in 12 towers. After remediation, the outbreak was declared over in late August 2025.

In response to this outbreak, the New York City Health Department enacted new citywide regulations requiring building owners to test cooling towers for Legionella bacteria “more frequently than anywhere else in the country.” (“New Legionella Testing Regulations for Cooling Towers  Go Into Effect Citywide as Summer Nears,” NYC Health, May 8, 2026). These changes probably contributed to the swift, successful outcome of the current outbreak in NYC.

Ongoing outbreak in New York City. New York City’s Upper East Side is facing a growing outbreak of Legionnaires’ disease that began in late June 2026. By the end of July, 84 confirmed cases and 5 deaths were reported. Live L. pneumophila was found in 51 cooling towers across 50 buildings, including the famed Guggenheim Museum of Art.

Public health officials moved quickly to eliminate the bacterium from impacted cooling towers. The fast testing, aggressive remediation, and transparent communication all contributed to what is seen as a model of how public health should work (Faust, J., “How New York Is Winning Against Legionnaire’s Disease,” MedPage Today, July 13, 2026). “In a time when federal programs have been hampered by cuts, making it harder to respond to new problems…New York City’s prompt actions in the current Legionnaires’ outbreak are a welcome reminder that public health, done by genuine experts, actually works.” (Emphasis added.) No new cases have been reported since mid-July.

The end of the golden era of the CDC. When members of the Pennsylvania American Legion began being sickened with a severe respiratory illness in July 1976, I was just beginning my career as an infectious disease specialist. As the death toll from the disease rose, I remember Congress pressuring the CDC to find the cause of this outbreak. Fortunately, there was a team of scientists at the CDC who were up to the task.

By January 1977, a team of researchers at the CDC led by microbiologist Joseph McDade discovered L. pneumophilia, the intercellular bacterium that caused the outbreak. (Dr. McDade was well positioned to discover the new pathogen as his previous research had focused on Rickettsia, another type of intracellular bacterium.) This breakthrough discovery heralded a long succession of CDC victories in the fight against what are now known as emerging infectious diseases.

The number of infectious diseases that have “emerged” since the 1970s is debated. Some say 140 or more, while the CDC and the World Health Organization suggest the number is closer to 40 or 50. No matter the number, in the past, the CDC led the effort to identify and combat these threats to public health. Sadly, that is no longer the case.

The CDC is now under the control of Robert F. Kennedy, Jr., the Secretary of Health and Human Services (HHS). In a November 2023 address to the Children’s Health Defense, the country’s largest anti-vaccine organization, Mr. Kennedy said: “We’re going to give infectious disease a break for about eight years.” Upon his confirmation as Secretary of HHS, Mr. Kennedy did just that. While the CDC may be giving infectious diseases a “break,” the current outbreaks of measles and Legionnaires’ disease in the U.S. show that the pathogens “didn’t get the memo.”