A 60-year-old man is diagnosed to be suffering from Legionnaires' disease after he returns home from attending a convention. He could have acquired it:

Infection normally occurs after inhaling an aerosol (fine airborne paicles) containing Legionella bacteria. Potential sources of such contaminated water include cooling towers used in industrial cooling water systems as well as in large central air conditioning systems. Note - Transmission to humans occurs when aerosols are created in manmade water supplies that harbor Legionella. Most outbreaks have occurred in or around large buildings such as hotels, factories, and hospitals with cooling towers or some other pa of an air-conditioning system as the spreading vehicle. Ref: Textbook of Microbiology By Ananthanarayan and Panicker, 6th Edition, Page 377 ; C.G., Ryan K.J. (2010). Chapter 34. Legionella. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.