Legionella in the Environment. Legionnaires' disease (LEE-juh-nares) 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. 1  Hot tubs.2  Cooling towers.3  Hot water tanks. 4  Large plumbing systems.egionnaires' disease (LEE-juh-nares) 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. 1  Hot tubs. 2  Cooling towers. 3  Hot water tanks. 4  Large plumbing systems.