The legionella bacteria is a natural inhabitant of water. Temperature is of great importance for the legionella proliferation. Especially temperatures between 40 and 50°C may lead to colonization of the bacteria.
Daikin e.g. does not use water in its air conditioning systems. So no harm for humans can be caused. In chilled water systems for industrial purpose, the legionella bacteria can not live because of the low temperature of the water in the closed circuit.