“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. The several workers at the county’s department of health came down with a fever and mild flu symptoms, but not pneumonia. 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. 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.”
Center for Disease Control and Prevention – CDC (https://www.cdc.gov/legionella/about/history.html)