How do hosts evolve to cope with the threat of infectious diseases? How do parasites that cause infectious diseases evolve to overcome host defences? These are the key questions that drive my research, which seeks to understand the continual evolutionary battle between hosts and parasites. Studying how host and parasites "co-evolve" not only has important implications for controlling infectious diseases, but also for answering fundamental questions about the natural world. All living organisms suffer from infectious diseases caused by parasites, many of which have a serious impact on host health and can be fatal. As such, hosts have evolved a variety of ways of coping with parasitism, from complex and expensive immune systems that recognise and eliminate microscopic pathogens, to grooming behaviour that removes external parasites. Likewise, parasites have evolved fascinating ways of avoiding or overcoming host defences (e.g. by secreting proteins that suppress immune responses), and even manipulating host behaviour to maximise transmission. Hosts and parasites exist in diverse multi-species communities, which have the potential to impact their evolution through increased competition for resources or changes in cost-benefit trade-offs associated with investing in defence and counter-defence traits. For example, predation may increase pressure on parasites by reducing host availability (resource competition) and may prevent a costly resistance mechanism from evolving if disease becomes rare (trade-offs). However, we currently know very little about how host-parasite coevolution is affected by the wider ecological community (e.g. other hosts and parasites, predators, prey, etc). The aim of this research programme is to understand the impact of species in the wider ecological community on the evolution of hosts and parasites. To achieve this, I will use mathematical models and computer simulations to reveal the underlying processes and mechanisms that affect how hosts and parasites co-evolve in the presence of other species. I will also collaborate with empiricists to test the new theory and use experimental observations to feed back and further develop the theory. I will first investigate how the structure of the ecological community affects co-evolutionary dynamics by analysing different networks of interactions. I will then use the knowledge that I have gained about the impact of the wider ecological community to examine the co-evolution of multiple host and parasite traits. This will allow me to answer questions such as: are parasites that infect multiple hosts more likely to be deadly? And when should hosts prioritise adaptive defences over innate defences? Finally, I will work with experimentalists to test and improve the theory by coevolving bacteria and viruses. My research will provide key insights into fundamental biological phenomena, such as how biodiversity is generated and maintained and the role that parasites play in the evolution of host mating behaviour. Although my work is theoretical, it has important implications for public health. For example, viruses can be used to treat antibiotic resistant infections (phage therapy), but it is crucial to understand how the presence of other microbial species affects bacterial evolution to maximise treatment efficacy. There are consequences for industry and conservation, as well, especially for predicting how human interventions or environmental changes affect the stability of natural ecosystems or managed populations.