[Morbidity: terminology, mechanisms of formation and classification]

In the 70-ies of the last century the evolution of the epidemic process doctrine resulted in developing population approach to morbidity analysis, in addition to the traditional focal approach. On the one hand, this led to the discussion on the possibility of applying such notion to non-infectious diseases and, on the other hand, to the discussion on the mechanisms of the development and formation of different manifestations in infectious diseases. In both cases the discussion was centered on the phenomenon of morbidity. The present article deals with the nature of morbidity as a phenomenon and the acceptability of general scientific approaches in the interpretation of seasonal and cyclic character of morbidity from the viewpoint of the wave process in an open system. On the basis of the data presented in this work the terms used in epidemiology in considering manifestations of the epidemic process are discussed. The improved classification of morbidity by qualitative and quantitative signs for infectious diseases is proposed.