"All animals are exposed to disease and infection, with the immune system providing crucial defences to stop animals becoming sick. Yet immunological variation is widespread, with the costs of immunity and differences in disease exposure being two potential causes. This proposal investigates how variation in disease pressure during early life influences immunity and what the consequences are for growth, a key developmental process against which immune investment may be traded-off. To do this I will study avian brood parasites called Honeyguides that exploit other species to raise their young. This exposes parasite nestlings to diverse antigens related to the range of hosts that might raise them. Honeyguides also destroy host eggs and kill host chicks which remain rotting in the nest, further exposing parasite nestlings to high disease pressure. I will ask if over evolutionary time this has selected for strong immune defences and if Honeyguides suffer less than host nestlings in terms of reduced growth when disease conditions are experimentally altered. Using a second brood parasite, where host-specific races parasitize hosts of different sizes, I will explore a related question: do growth conditions during development affect immune function in adulthood? Finally, using modern metagenomic approaches I will examine how brood parasites and hosts acquire their gut microbiota and how disease exposure might shape this microbial community, providing important data on the microbiomes of wild birds that is currently lacking. Combining immunology, ecology, microbiology and evolutionary biology, this study will provide key insights into how disease exposure influences immunity and growth, and will be relevant for researchers interested in disease ecology, coevolution and the broad implications of disease for human and animal health. The project also offers excellent training prospects and will strengthen my qualities as a mature, independent researcher."