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Most studies of insect-pathogen interactions consider the direct interaction between one disease agent and one species of host. However, given that hosts are subject to challenge from many pathogen/parasite species, mixed infections are probably common. In this study, using the desert locust and two species of fungal entomopathogen, we show how mixed infection with a largely avirulent pathogen can alter the virulence and reproduction of a second, highly virulent pathogen. We find that two strains of the avirulent pathogen vary in their interaction with the virulent pathogen, depending on the order of infection and environmental conditions. We propose that avirulent pathogens, which have largely been overlooked to date, could play a significant role in host-pathogen dynamics, with implications for biological control and evolution of virulence.