Cooling tower Legionella control is necessary so the Legionella bacteria does not grow and spread within the towers. Legionella bacteria are a type of bacteria that can cause Legionnaires' disease, a serious lung infection. Cooling towers are one of the most common places where Legionella bacteria can grow and spread. This is because cooling towers are large, open structures that contain water and a warm, humid environment, which are ideal conditions for Legionella bacteria to grow.