History, Burden, and Trends
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.
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.
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. More illness is usually found in the summer and early fall, but it can happen any time of year.
Read about various illnesses, including Legionnaires’ disease, in CDC’s first estimates of the impact of waterborne disease in the United States.