History, Burden, and Trends

Infections with Legionella bacteria include Legionnaires’ (LEE-juh-nares) disease and Pontiac fever. Legionnaires’ disease is a serious type of pneumonia (lung infection), while Pontiac fever is a milder infection that usually gets better without medical care.


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 that eventually became known as Legionnaires’ disease.

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 Legionnaires’ disease outbreak in Philadelphia that public health officials were able to show that Legionella causes both diseases.

Burden and Trends

The number of cases reported to CDC has been on the rise since 2000. Health departments reported nearly 10,000 cases of Legionnaires’ disease in the United States in 2018. However, because Legionnaires’ disease is likely underdiagnosed, this number may underestimate the true incidence. A recent study estimated that the true number of Legionnaires’ disease cases may be 1.8–2.7 times higher than what is reported.2 More illness is usually found in the summer and early fall, but it can happen any time of year.

Burden of Waterborne Disease

Read about various illnesses, including Legionnaires’ disease, in CDC’s first estimates of the impact of waterborne disease in the United States.


  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.
  2. Collier SA, Deng L, Adam EA, et al. Estimate of burden and direct healthcare cost of infectious waterborne disease in the United States. Emerg Infect Dis. 2021;27(1):140–9.
Graphic: Montage of images, including Pennsylvania state seal, Time magazine cover, Newsweek magazine cover and the American Legion badge

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

Legionnaires' disease is on the rise in the US 2000-2018

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In the United States, the rate of reported cases of Legionnaires’ disease has grown by nearly nine 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.