Where I live there is a growing number of cases of Legionnaires' disease, which can be fatal. Elderly people and those with respiratory problems are especially vulnerable.
Infection is mainly through breathing in an aerosol, for example in a shower.
The bacteria are naturally present in water supplies and thrive in temperatures between 20C and 50C. I would imagine that many people have turned down their hot water thermostats unaware that this can promote rapid growth of the bacteria.
Official advice is to keep the hot water tank at 50C or hotter, and at the design stage, to avoid long lengths of hot water pipe, which can cool down sufficiently to promote the growth of the bacteria.
Some water heating systems periodically raise the temperature in the hot water tank above the set temperature, specifically to kill off any bacteria in the tank on a regular basis. I am not sure if this is a generalised feature or just from one manufacturer (Vaillant).