Legionnaires' disease, the pneumonic form, has an incubation period of 2 to 10 days (but up to 16 days has been recorded in some outbreaks). Initially, symptoms are fever, loss of appetite, headache, malaise and lethargy. Some patients may also have muscle pain, diarrhoea and confusion.

The first signs of Legionnaires' disease—a severe form of pneumonia caused by the Legionella bacteria—typically appear between two to ten days after exposure. The initial symptoms are often flu-like and include a high fever (often 104°F or higher), chills, a persistent dry cough, and significant muscle aches. Many patients also experience a severe headache and a general sense of tiredness or malaise. Unlike a standard cold, Legionnaires' can also present early with gastrointestinal issues such as nausea, vomiting, and watery diarrhea. As the infection progresses into the lungs, shortness of breath and chest pain become prominent. In some cases, particularly in older adults, the first noticeable sign may be confusion or other mental changes (encephalopathy) due to the systemic nature of the infection. Because these symptoms mimic other types of pneumonia and the flu, medical professionals in 2026 rely on specific urine or phlegm tests to confirm the diagnosis, which is crucial because the disease requires specific antibiotics to be treated effectively.