Legionnaire’s disease is a water-borne bacterial pathogen that grows intracellularly in amoebae (and macrophages, due to their similar structure).
Originally discovered after an outbreak of pneumonia at a hotel hosting an American Legion conference, Legionella is frequently found in hot water systems that are not at the proper temperature, as well as poorly maintained air conditioning systems.
Legionella can be treated with fluoroquinolone antibiotics, with low mortality rates now the infection is understood.
To prevent Legionella from growing, biocides can be added to water distribution systems (such as chlorine dioxide). Hot water systems should be kept >60C to prevent growth.