Many infectious diseases of recent concern, including malaria, cholera, plague, and Lyme disease, have emerged from complex ecological communities, involving multiple hosts and their associated parasites. Several of these diseases appear to be influenced by human impacts on the environment, such as intensive agriculture, clear-cut forestry, and habitat loss and fragmentation; such environmental impacts may affect many species that occur at trophic levels below or above the hostcommunity. These observations suggest that the prevalence of both human and wildlife diseases may be altered in unanticipated ways by changes in the structure and composition of ecological communities. Predicting the epidemiological ramifications of such alteration in community composition will require strengthening the current union between community ecology and epidemiology.