Water of open reservoirs significant amount of nutritious substances, that promotes development of various microorganisms in it, contains. Depending on the sizes they are subdivided on ultramicrobs, bacteria and multicell organisms (mushrooms, seaweed). The small amount of substance with infected bacteria is enough for distribution of infectious diseases through water. Bacteria has extreme activity and their high sensitivity to change of conditions of existence. Microbs are divided into three groups by the way of eating: autotrophic, which receive energy, necessary for life, from photosynthesis (mastering of carbonic acid) or hemosynthesis (oxidation of sulfur, iron (II) etc.); heterotrophic, requiring ready organic substances for construction of their organisms; paratrophic (parasites), requiring in alive fiber. As breath, which is carried out by all surface of a crate, microbes are divided on aerobes and anaerobes. First develop only at an easy approach of oxygen, second can live only at absence of oxygen of air. There is also group facultative unaerobes, capable to develop at small access of oxygen. Natural waters alongside with useful microbes, participating in the process of self-cleaning of reservoirs, contain also harmful, pathogenic microorganisms that are the activators of infectious diseases - typhus, choleras, Siberian ulcer. The viruses are ultramicrobes, which are so small, that pass through filters detaining usual bacteria. Absence of cell structure, the simplicity of chemical structure, originality of an exchange of substances (they are the parasites of an alive crate of animals and plants) are characteristic for viruses. They collect and pass the certain cycle of development in the appropriate alive cells. Action of many antibiotics does not effect them. On a cold the filtered viruses keep their activity during many years. The drying does not kill them. At high temperature (about 90 0C) the viruses perish; some of them do not maintain even 55 0C.
Except for bacteriological research biological one is also carried out. It consists in study of water organisms.
