Wildlife diseases in avian populations present both a threat to species conservation and human health. This provides an impetus to study the parasitic agents of these diseases, opening the door to addressing fundamental questions pertaining to disease ecology and evolution. In this thesis, I explore the impact of avian diseases on natural populations from a variety of perspectives. These include exploring interactions between parasites and: their hosts, the wider abiotic environment, and other co-infecting parasites. Ultimately, my aim has been to further our understanding of the ecology of host-parasite interactions. In Chapter 2, I investigate the introduction of avian pox (Avipoxvirus) and avian malaria (Haemosporida) to novel host populations. In the Galápagos I find that despite previous reporting, the under-surveyed island of San Cristóbal remains free of avian malaria. However, avian pox is found to be associated with urbanised areas and strongly influenced by climate, suggesting future anthropogenic change will increase the disease burden of endemic species. In Chapter 3, I shift focus to a more well established host-parasite system in order to investigate a key determinant of infection dynamics; that of parasite host-breadth. Specifically, within a diverse community of Leucocytozoon blood parasites infecting European Paridae, I study interactions between host-specialists and generalists detected as negative associations between hosts. I find that, as predicted, specialist Leucocytozoon parasites are not only more prevalent and induce reproductive costs in their hosts, but also appear to be superior competitors, able to facilitate the exclusion of community generalists. In Chapter 4, I address the difficulty of detecting infection costs in wild animal populations. I study the ubiquitous Plasmodium relictum and the costs of infection during the over-wintering period for great tits (Parus major). By studying a physiologically costly behaviour - flight, I aim to detect whether infection costs are realised as trade-offs against this vital host trait. This chapter provides evidence that hosts can buffer against infection in some traits but not others while highlighting the continued complexity of measuring infection costs in natural populations. Lastly, in Chapter 5, I present the vital importance of considering methodological approaches in studies of blood parasites. I compare the effects of both storage medium and storage duration on our ability to detect and quantify infection in samples stored for a three-year period. I find strong evidence that both storage medium and duration can influence these measures, highlighting that these choices should be critical consideration in ecological studies of avian Haemopsoridia. Together this body of work provides novel insight into a number of complexities and necessary considerations to measuring host realised costs of avian blood parasites. Furthermore, my findings provide the first in-depth survey of avian disease emergence on San Cristóbal, Galápagos, and highlights the further complexity anthropogenic change will likely introduce to host-parasite interactions in the future.