Research advances in the last 10 years have established the crucial, yet frequently hidden role that pathogens and parasites play in ecological systems. Conventional host-parasite models assume that all individuals in a host population are equally susceptible and equally responsible for pathogen transmission – but more recent empirical data suggest that host immune function plays an important role in determining whether an exposed individual will become infected and whether it will be an efficient disease transmitter. At the same time, immune function can be shaped by an organism’s life history strategy and also by external conditions, with environmental changes impacting the ability of organisms to resist infection. This in turn affects whether an epidemic becomes established in a host population, how rapidly it spreads, and whether or how long it persists. This symposium brings together leading researchers in disease ecology, physiological ecology, and the growing field of ecological immunity. Focusing on host organisms that include coral, mice, birds, butterflies, plants, and humans, speakers offer new insights on the importance of individual host variation to advance our understanding about the ways disease epidemics emerge and unfold. These advances have broad ramifications ranging from the way biologists model infectious diseases to how endangered species can be managed in response to threats from pathogens and from global environmental change.