Legionella are dangerous because they can cause serious respiratory diseases such as Legionnaires’ disease and Pontiac fever. Legionnaires’ disease can lead to severe pneumonia, which in some cases is fatal, especially in older people, smokers and those with weakened immune systems. The bacteria thrive in warm water environments such as hot water pipes, air conditioning systems and hot tubs and are spread by inhaling water droplets contaminated with Legionella. Control and prevention are difficult because Legionella live in biofilms in water systems and large, complex water systems present particular challenges.