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History, Burden, and Trends

Español: Historia y patrones de la enfermedad

Graphic: Montage of images, including Pennsylvania state seal, Time magazine cover, Newsweek magazine cover and the American Legion badge

Legionellosis, which includes Legionnaires’ (LEE-juh-nares) disease and Pontiac fever, is a respiratory disease caused by a type of bacteria called Legionella.


Legionella was discovered after an outbreak in 1976 among people who went to a Philadelphia convention of the American Legion. Those who were affected suffered from a type of pneumonia (lung infection) that eventually became known as Legionnaires’ disease.

The pictured magazine covers feature the work of public health professionals in 1976 as they raced to trace the origin of the first documented outbreak of Legionnaires’ disease in Philadelphia, Pennsylvania.1

The first identified cases of Pontiac fever occurred in 1968 in Pontiac, Michigan, among people who worked at and visited the city’s health department. It wasn’t until Legionella was discovered after the 1976 outbreak in Philadelphia that public health officials were able to show that the same bacterium causes both diseases.

This graph shows reported legionellosis incidence (per 100,000 persons) in the United States from 2000-2014.

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In the United States, reported cases of Legionnaires’ disease have grown by nearly four and a half times since 2000.

It is unclear whether this increase represents artifact (due to increased awareness and testing), increased susceptibility of the population, increased Legionella in the environment, or some combination of factors.


Burden and Trends

The number of cases reported to CDC has been on the rise since 2000. Health departments reported about 6,100 cases of Legionnaires’ disease in the United States in 2016. However, because Legionnaires’ disease is likely underdiagnosed, this number may underestimate the true incidence. More illness is usually found in the summer and early fall, but it can happen any time of year.



  1. Magazine story covers related to the 1976 outbreak of Legionnaire’s disease. From Centers for Disease Control and Prevention (CDC) Public Health Library, #1185. Copyright 1976 by CDC. Reprinted with permission.