Legionella is ubiquitous in the environment and is found in hot and cold water taps, creeks, ponds, and surrounding soil. Potting soil has also been associated with cases of Legionellosis, though the mechanism is unknown. Legionella does not transmit from person to person.
For water, the route of exposure is through aerosolization or inhalation. Exposure has been associated with hot water systems, air conditioning cooling towers, evaporative condensers, mist machines, decorative fountains, respiratory therapy devices, humidifiers, hot tubs, and whirlpool spas.