It is likely that Legionella bacteria will enter your water system at some point. It can double in numbers, HOURLY, if conditions are right for it.
‘Conditions’ include nutrient and temperature. Nutrient can be anything organic -eg. rust, algae, sediment, insects etc, etc, etc…. temperatures between 20 ºc and 45 ºc encourage proliferation, 37 ºc is peak breeding temperature.
A person’s lungs are organic, normal body temperature is 37 ºc.
This is why it kills.
Water hygiene is an essential part of a Legionella Control System. Better to assume that legionella will arrive, best to avoid giving it a good home.