Abstract: According to the concept of apparent competition, one species can exclude another indirectly by a shared parasite, without the need for direct interactions. Here, we discuss how immunity influences host fitness in this context. On an ecological level, lower immunity and the maintenance of pathogens can help a species to invade a competitor’s space and to drive it extinct. In other parameter regions, maintenance of parasites can prevent invasion and ensure co-existence. Despite these advantages, immunity is expected to evolve to-wards pathogen elimination because the effects of intra-specific competition outweigh the helpful role of parasites in intera-specific competition.