Legionnaires' disease is a severe form of pneumonia caused by the bacterium Legionella pneumophila. It is typically contracted by inhaling contaminated water droplets from sources like air conditioning systems, hot tubs, or plumbing systems. Symptoms include cough, fever, chills, and muscle aches, often appearing 2 to 10 days after exposure. The disease is named after an outbreak at a American Legion convention in 1976, where many attendees fell ill. While it can affect anyone, those with weakened immune systems, older adults, and smokers are at higher risk. Treatment usually involves antibiotics, and most people recover with proper care.