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History and Disease Patterns

Español: Historia y patrones de la enfermedad

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


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

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.

Disease Patterns

Illness caused by Legionella continues to be detected, now more than ever. About 5,000 cases of Legionnaires’ disease are now reported each year in the United States. 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.