A 45-year-old man who is a heavy smoker complains of sudden onset of high fever and chills, nonproductive cough, and chest pain. An X-ray film of the chest shows bilateral, diffuse, patchy infiltrates. The patient reports that a number of similar cases have occurred recently in the building where he works. He responds to antibiotics and recovers. Which of the following is the most likely a reservoir for the microorganism that is responsible for this patient's respiratory tract infection?

Legionella pneumophila causes a pneumonia that ranges from mild to a severe life-threatening, necrotizing pneumonia referred to as "Legionnaire disease." The bacterium is found in natural bodies of fresh water and survives chlorination, allowing it to proliferate in cooling towers, water heaters, humidifiers, and ventilation systems. Legionella pneumonia begins when microorganisms enter alveoli, where they are phagocytized by resident macrophages. Bacteria multiply within macrophages and are released to infect new macrophages. The disease presents as an acute bronchopneumonia, with a diffuse and patchy pattern of infiltration. None of the other situations creates suitable conditions for bacteria to multiply.Diagnosis: Legionnaire disease