Analysis of morpho-ecological adaptations of parasites showed that their necessary and sufficient characteristic is their inhabiting the host environment. Integration with the environment is achieved 1) by adaptations towards a particular habitat (first order environment) at certain phases of ontogenesis, and 2) by adaptations to the aggregate of hosts, their ecology and factors regulating the numbers of parasites (second order environment) at the level of their entire life cycle. The evolutionary progress of parasites is sustained by the increase in their independence from extrinsic factors not by means of general organization complication and integration (as in free-living animals), but by elimination of free-living developmental stages and by increase of integration with the host environment. The basis of the latter is formed by ecological adjusting of parasites to the environment. It is achieved by syncytial transformation of the covers and the use of communication means analogous to those of host cells. The consequence of host-parasite integration is the transition of parasites from resource consumption to the "management" of the environment. It is manifested by their influence on defensive reactions and other functions of the organism, as well as on its behavior and ecology. Parasitism may turn into other forms of coexistence (commensalism, mutualism) in the course of coevolution, as a result of dialectical overcoming of antagonistic interactions of the two partners.