Legionella (Legionella pneumophila) is a bacterial pathogen that, in humans, causes a rare and severe form of pneumonia. The bacteria typically enter the lungs via aerosolized water.

In man made environments, the pathogen can multiply in water sources susceptible to stagnation and contamination, such as cooling towers, evaporative coolers, humidifiers, whirlpool spas, decorative fountains, water heating systems, showers, ice machines, and misting systems.

The bacteria attack the immune response system by infecting white blood cells, destroying them, and multiplying. There is no preventive vaccine; treatment is via antibiotics. Hospital acquired legionella pneumonia has a fatality rate of 28% and can rise as high as 50% if treatment is delayed by misdiagnosis.