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Legionellosis outbreaks occur when two or more people are exposed to Legionella and get sick in the same place at about the same time. Legionellosis is not spread from person to person. People get it when they breathe in a mist or vapor (small droplets of water in the air) that is contaminated with Legionella.

Outbreaks are commonly associated with buildings or structures that have complex water systems, like hotels and resorts, hospitals, and cruise ships. The most likely sources of infection include hot tubs, cooling towers (air-conditioning units for large buildings), and water used for drinking and showering. Legionellosis outbreaks can be difficult to identify, especially if people travel to a common location, are exposed to Legionella, and then return home before becoming sick.

State and local health departments take the lead in investigating outbreaks and implementing control measures to remove Legionella from the water identified as the source of infection. CDC is only involved in legionellosis outbreak investigations when state and local health departments request additional assistance. State and local health departments are the best source of information for a specific outbreak.