Interactions between hosts and pathogens have consequences not only at the individual level but also cascading up through populations, communities, and ecosystems. In this course, we will look at infectious disease through the lens of ecology. First, we will consider infected hosts as ecosystems, focusing on ecological interactions within hosts—both between microorganisms and between pathogens and the host immune system. Next, we will investigate disease dynamics within and between populations, including the emergence of new diseases and the dynamics of vector-borne disease systems. Simple models of disease transmission and spread will be introduced. Finally, we will explore the larger impacts of disease on biological communities and entire ecosystems, looking at topics such as the relationship between disease and biodiversity and the surprising ways in which disease can affect ecosystem structure and function. Examples will be drawn from plant, wildlife, and human disease systems.