Legionella are rod-shaped bacteria that can cause legionellosis. The most common pathogen is Legionella pneumophila. Legionellosis includes Legionnaires’ disease, a severe sometimes life-threatening pneumonia, and Pontiac fever, a less severe infection without pneumonia.
As legionella naturally occur in freshwater habitats, they can easily enter potable water systems. Under favourable conditions, legionella can settle and propagate within biofilms in potable water tanks, pipes and installations. There is an increased risk for legionella propagation at water temperatures between 25 and 45 °C.
If legionella-containing warm water is nebulised and distributed via showers, air conditioning or cooling towers, there is an increased risk of infection for people inhaling the aerosols.
Conservative methods of eliminating legionella from potable water systems such as thermal disinfection and water or water/air flushing have shown to be ineffective because they do not reach and remove the biofilm in which legionella reside. The remaining organic material provides an optimal breeding ground for the re-growth of the remaining legionella after the measure.