Signs and symptoms include cough, shortness of breath, high fever, muscle pains, and headaches.

[4] Risk factors for infection include older age, a history of smoking, chronic lung disease, and poor immune function.

Distinguishing Legionnaires' disease from other types of pneumonia by symptoms or radiologic findings alone is difficult; other tests are required for definitive diagnosis.

[citation needed] Legionnaires' disease is usually spread by the breathing in of aerosolized water or soil contaminated with the Legionella bacteria.

[26] To minimize risks of bacterial growth, the American Society of Heating, Refrigerating and Air-Conditioning Engineers' 1988 ASHRAE Standard 188 and subsequent ASHRAE Guideline 12-2000 increased recommended hot water generation and storage temperatures to 135–140 °F (57–60 °C) with minimum distribution temperatures of 124 °F (51 °C).

[1] In the built environment, central air conditioning systems in office buildings, hotels, and hospitals are sources of contaminated water.

[1][28] The bacteria may also be transmitted from contaminated aerosols generated in hot tubs if the disinfection and maintenance programs are not followed rigorously.

Virulent strains of Legionella kill macrophages by blocking the fusion of phagosomes with lysosomes inside the host cell; normally, bacteria are contained inside the phagosome, which merges with a lysosome, allowing enzymes and other chemicals to break down the invading bacteria.

Even in countries with effective health services and readily available diagnostic testing, about 90% of cases of Legionnaires' disease are missed.

This is partly due to the disease being a relatively rare form of pneumonia, which many clinicians may not have encountered before, thus may misdiagnose.

A further issue is that people with legionellosis can present with a wide range of symptoms, some of which (such as diarrhea) may distract clinicians from making a correct diagnosis.

[34] Although the risk of Legionnaires' disease being spread by large-scale water systems cannot be eliminated, it can be greatly reduced by writing and enforcing a highly detailed, systematic water safety plan appropriate for the specific facility involved (office building, hospital, hotel, spa, cruise ship, etc.

[16] To inform the water safety plan, the undertaking of a site specific legionella risk assessment is often recommended in the first instance.

Current treatments of choice are the respiratory tract quinolones (levofloxacin, moxifloxacin, gemifloxacin) or newer macrolides (azithromycin, clarithromycin, roxithromycin).

Tetracyclines and erythromycin led to improved outcomes compared to other antibiotics in the original American Legion outbreak.

[1] During outbreaks, prophylactic antibiotics have been used to prevent Legionnaires' disease in high-risk individuals who have possibly been exposed.

[1] The mortality at the original American Legion convention in 1976 was high (29 deaths in 182 infected individuals[37]) because the antibiotics used (including penicillins, cephalosporins, and aminoglycosides) had poor intracellular penetration.

[medical citation needed] The fatality rate of Legionnaires' disease has ranged from 5–30% during various outbreaks and approaches 50% for nosocomial infections, especially when treatment with antibiotics is delayed.

[38] Hospital-acquired Legionella pneumonia has a fatality rate of 28%, and the principal source of infection in such cases is the drinking-water distribution system.

[39] Legionnaires' disease acquired its name in July 1976, when an outbreak of pneumonia occurred among people attending a convention of the American Legion at the Bellevue-Stratford Hotel in Philadelphia.

Environmental studies continue to identify novel sources of infection, leading to regular revisions of guidelines and regulations.

[45] Between 1995 and 2005, over 32,000 cases of Legionnaires' disease and more than 600 outbreaks were reported to the European Working Group for Legionella Infections.

[16] Improvements in diagnosis and surveillance in developing countries would be expected to reveal far higher levels of morbidity and mortality than are currently recognised.

[46] During the COVID-19 pandemic, some researchers and organisations raised concerns about the impact of the COVID-19 lockdowns on Legionnaire's disease outbreaks.

[49] An outbreak is defined as two or more cases where the onset of illness is closely linked in time (weeks rather than months) and space, where a suspicion or evidence exists of a common source of infection, with or without microbiological support (i.e. common spatial location of cases from travel history).