Legionella are waterborne bacteria that can proliferate under favourable conditions (temperatures between 25 and 50° Celsius) in drinking water installations in buildings or in  evaporative cooling systems. For humans, however, it only becomes dangerous when Legionella enter the lungs through nebulisation when inhaled, e.g. when showering. This can lead  to legionellosis, a dangerous form of pneumonia. It is a notifiable but non-human-tohuman infectious disease which, in the worst case, can lead to death.

The German Robert Koch Institute registers up to 1,000 reported cases of legionellosis per year in Germany, of which approximately 5% ended in death. Many sources assume a very high number of unreported cases in the range of 15,000 to 30,000 diseases a year, which were not recognized as such and can even be assumed to have a mortalty of up to 10%.