The bacterium that causes Legionnaires disease loves warm water. People are exposed when they use the water. The bacterium: Legionella, is now the number one cause of waterborne outbreaks: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6235a3.htm
Here is a description of the disease:
http://www.mayoclinic.org/diseases-conditions/legionnaires-disease/basics/definition/con-20028867
WHO has prepared a document on risk factors and control of the bacteria: http://www.who.int/water_sanitation_health/emerging/legionella.pdf
"The document also identifies necessary measures to prevent, or adequately control, the risk of exposure to Legionella bacteria for each particular environment. Outbreaks of legionellosis generally cause a high level of morbidity and mortality in the people exposed; therefore, the suspicion of an outbreak warrants immediate action. "
"…naturally occurring L. pneumophila survived and multiplied in water at temperatures between 25 ºC and 45 ºC, with an optimal temperature range of 32–42 ºC. The study also found that legionellae were most commonly isolated at temperatures between 35 ºC and 45 ºC, with the greatest increase in viable counts occurring between 37 ºC and 42 ºC (Wadowsky & Yee, 1983; Schulze- Robbecke, Rodder & Exner, 1987). As the temperature falls below 37 ºC, the bacteria’s reproductive rate decreases and there is little or no increase in numbers of bacteria below 20 ºC. Therefore, to prevent Legionella infection, the recommended temperature for storage and distribution of cold water is below 25 °C, and ideally below 20 °C. Recent laboratory studies of mutant Legionella strains show that the bacteria may grow below 20 ºC under certain conditions (Soderberg, Rossier & Cianciotto, 2004). Legionella will survive for long periods at low temperatures and then proliferate when the temperature increases, if other conditions allow."
Stay safe!
Tall