What Is Legionnaires’ Disease?

In 1976, the United States celebrated its bicentennial. Jimmy Carter beat Gerald Ford in the presidential election. And in Philadelphia, dozens of attendees at an American Legion conference fell ill with a mysterious respiratory disease. Out of 2000 conference guests, 221 people were infected and 34 died. It cause was a then-unknown type of bacteria that was eventually named for its victims. But what is Legionnaires’ disease, exactly?

Legionellosis is any infection from a genus of bacteria called Legionella. The most famous of these infections is the atypical pneumonia called Legionnaires’ disease, which is usually caused by a species called Legionella pneumophila. The good news is that most people who are exposed to L. pneumophila or the less-frequently seen species L. longbeachae, L. feeleii, L. micdadei, and L. anisa don’t usually get sick. If you do contract Legionnaires’ disease, it’s treatable.

Cases in the U.S. account for about 10 percent of the global prevalence of Legionnaires’ disease, and between 8000 and 18,000 Americans are hospitalized with it each year. Most healthy people can and do recover through treatment with antibiotics, though there is a risk of lifelong complication from an infection left untreated too long. People most at risk for a serious or deadly outcome are immunocompromised, over age 50, smokers, or suffering from chronic lung illnesses such as chronic obstructive pulmonary disease (COPD).

The sooner treatment begins, the better the chances are of recovery. One reason treatment might be delayed is that Legionnaires’ disease’s symptoms—high fever, cough, shortness of breath, muscle aches or headaches—are similar to other respiratory illnesses, such as influenza or other types of pneumonia. While the symptoms usually appear two to 10 days after exposure, it can take as long as two weeks to become symptomatic.

Fortunately, Legionnaires’ disease cannot be transmitted from person to person. Instead, people are infected by breathing in airborne mist from contaminated water sources, such as improperly cleaned hot tubs, ventilation systems, or large plumbing systems. In the 1976 case, the bacteria was eventually found in the cooling towers of the Bellevue-Stratford Hotel where the conference took place, and it was rapidly distributed throughout the building via the air conditioning system.

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A version of this story was published in 2015; it has been updated for 2024.