Legionellosis is an infection caused by the Legionella bacteria, which are commonly found in water systems, such as hot tubs, cooling towers, and plumbing systems. People can become infected by inhaling aerosolized water droplets containing the bacteria. The disease can manifest as a mild flu-like illness or a more severe form known as Legionnaires' disease, which can lead to pneumonia. 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. Prompt diagnosis and treatment with antibiotics are essential for recovery.