The object of the module is to examine, from a evolutionary and ecological perspective, the complex interactions between parasites/diseases and their hosts and to show how the selection pressures that each side of these interactions impose lead to coevolutionary processes. We will take an overview of the role that such parasitic interactions may have played in the development of key biological traits, such as the evolution of sexual reproduction, and their current role in sexual selection. The module will include traditional parasitology (to set the scene and understand the complexity of the interactions), introducing the major groups of parasites and their hosts. We will examine the role of parasites and host-parasite interactions in evolution, drawing examples from conservation, behaviour, current research, theoretical predictions and models.