Parasite adaptations to within-host competition

Competition between parasite species or strains within hosts is a major evolutionary force in infections. In response, parasites exhibit a diverse array of strategies that improve their chances of growth or reproduction over competitors. This Review describes three types of competition that parasites face (exploitation, apparent and interference), identifies successful strategies for confronting these and discusses whether these strategies are true adaptations to competition. Although many studies of multiple infections have focused on disease outcomes (e.g. virulence), rather than on the particular parasite strategies that have adapted in response to the ensuing competitive interactions, these strategies are ultimately responsible for shaping disease outcomes of interest. A better understanding of parasite adaptations to competitive interactions will have important public health implications.