Bacteria are single-celled microorganisms that mostly live together in the form of biofilms. Bacteria multiply by simple cell division. The warmer the water, the faster bacteria multiply. From approx. 10°C onwards, the bacteria start to multiply, and at 25°C, for example, the growth rate is already tripled. But many bacteria in the fresh water system are harmless to humans. Ultimately, it depends on the type and number of bacteria, because in addition to harmless species, there are also pathogens that can definitely harm the human organism. Legionella and E.Coli (intestinal bacteria) are among the most common dangerous pathogens here. With disinfection of the water, these can be killed and rendered harmless.