Various interpretations of the parasitism phenomenon exist. In this work data supporting the ecological understanding of its nature are presented. For parasitic species formed in the process of evolution the host organism has become the habitat (nutrition, multiplication), i.e., the environment, where interactions between the host and parasites are governed by ecological regularities. The consequences of this interactions are different (disease, asymptomatic infection), being secondary they reflect the result of concrete interactions. For this reason parasitism should not be identified in terms of only one consequence of such interaction--the development of pathologic processes.