Hi, Legionnaire pneumonia is caused by a type of bacteria called Legionella. The Legionella bacteria are found naturally in the environment, usually in water. The bacteria grow best in warm water, like the kind found in Hot tubs Cooling towers Hot water tanks Large plumbing systems Decorative fountains Centralised air conditioning People get Legionnaires disease when they breathe in a mist or vapor (small droplets of water in the air) containing the bacteria. One example might be from breathing in droplets sprayed from a hot tub that has not been properly cleaned and disinfected. The bacteria are not spread from one person to another person.