Legionnaires' disease is a severe form of pneumonia caused by the bacterium Legionella. This bacterium thrives in warm water environments, such as hot tubs, cooling towers, and large plumbing systems. People can become infected by inhaling mist or vapor containing the bacteria. Symptoms of Legionnaires' disease include cough, shortness of breath, fever, and muscle aches. It primarily affects older adults, smokers, and individuals with weakened immune systems. Early diagnosis and treatment with antibiotics are crucial for recovery, as the disease can lead to serious complications if left untreated.