[Parasitism--the existence of organisms as a component of parasitic systems]

Attempt to give a definition of the parasitism phenomenon from the symbiotological point of view is proposed. Parasites are the organisms, existence of which is possible only in the parasitic systems of any degree of complexity. Outside the parasitic system the parasite is not capable to realize its main vital function including feeding, dispersion of reproduction. The parasitism as the biological phenomenon could be defined as the existence of organisms in the structure of parasitic systems.