Polyparasitism appears to be the rule, rather than the exception, both in populations and in individuals in the developing countries of the world. Thus, polyparasitism represents coendemicity in the epidemiologic sense and simultaneous infections in individual patients in the clinical sense. The effects of polyparasitism are often clinically inapparent. In some situations, however, combined infections may exacerbate clinical manifestations. Coexistent infections may also, under some circumstances, suppress disease symptoms. The possibility of either synergistic or antagonistic effects must therefore be considered in planning public-health intervention intervention programs, and the priorities or strategies selected may need to be altered accordingly. There are few available data at present that are suitable for evaluation of the real consequences of polyparasitism, in part because of the many confounding variables involved and the lack of prospective studies. Therefore, future intervention programs should be accompanied by an epidemiologic research component designed to detect clinical or laboratory changes in parasitic and other infections or in host responses.