The dynamics of infectious disease are driven by the fundamental processes that mediate host–pathogen interactions. A basic understanding of the mechanisms underlying these interactions is essential for disease ecologists regardless of their scale of inquiry. This chapter covers the terms and concepts commonly used in ecological studies of infectious disease across levels of organization and scales of inquiry, from the individual host organism to host populations and multispecies communities. When applicable, aspects that are unique to birds and their biology are highlighted. The between-host processes discussed in the beginning of the chapter arise from the within-host processes between the pathogen and the host’s immune system. These processes are then used as a framework to introduce the basics of epidemiological modeling and the population-level disease dynamics. The chapter is not meant to be exhaustive but, instead, to provide a common foundation for readers approaching this topic from unique backgrounds. Given the transdisciplinary nature of avian infectious disease ecology, many of the terms used have multiple meanings assigned to them that are taxon- or discipline-specific. Such variation in key terminology is, in large part, a consequence of the transdisciplinary and multiscaled approaches inherent in studying host–pathogen–vector–environment interactions.