Pathology describes the effects of the parasite on the host, the morphologic and functional changes produced, and the host response. While pathology implies a static description, pathogenesis refers to the dynamic events and interactions. An overall successful host-parasite interaction occurs in the life cycle of many parasites and generally depends on development of immunity by the host. The infective agent may be eliminated or persist as a chronic inactive infection. Some parasites have learned to circumvent some of the immune mechanisms that tend to contain or eliminate them.
Examples of unsuccessful interactions include accidental parasitisms where usually the invader, and occasionally the host, is rapidly killed. In contrast to this, successful parasitism implies the ability to persist and to reproduce in a host without giving rise to lesions that prevent survival as a species of either parasite or host.