Legionnaires' disease is the common name for illnesses caused by Legionnaires' disease bacteria (LDB). Legionnaires' is an infection of the lungs that is a form of pneumonia.
Sources of Legionnaires' disease can include almost any warm water system or device that disperses water, particularly as aerosols, sprays, or mists. Devices that can amplify LDB include cooling towers, evaporative condensers, humidifiers, holding tanks, potable water plumbing systems and heaters, and pipes that contain stagnant warm water. Transmission of Legionnaires’ disease occurs when microscopic airborne droplets of water that contains LDB are inhaled by an at-risk host (a person who lacks immunity or resistance).
This factsheet from NYCOSH contains more information about Legionnaires' disease: how it's transmitted, its symptoms, treatment, who is most at risk, infection control measures, and reporting.