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Parasite species assemblages currently are thought to range from isolationist to interactive, their dynamic properties being related to the number of species and types of hosts involved. The literature contains few experimental tests of this concept, however, and many of the host/parasite systems studied to date are not amenable to experimental manipulation. In this review, the presence of a parasite species, in a sample of host individuals, is considered to be an evolutionary phenomenon, but the parasite's population structure is considered to be an ecological one. Studies that allow evaluation of these two influences are comparative in nature and include data from a series of homogeneous samples of host populations. A lottery model is presented, in which hosts acquire their assemblages of parasites by Monte Carlo type sampling from multiple kind arrays; the major structuring influence is the relative probability of becoming infected by various parasite species. Claims of parasite species interaction need to be supported by studies showing departures from the predictions of this model. The species density and infra-assemblage diversity index distributions are recommended as quantitative tools useful in such work.