The within-host demography of pathogens, or pathogen population growth, spread, and clearance within our bodies, shapes their ecology and evolution, driving outcomes of public health relevance such as the emergence of drug resistance. The cross-scale dynamics of infectious disease drives its impact on human health, yet empirical studies tend to either focus on the within-host aspect or the between host aspect (transmission) — the Metcalf Lab has developed key methods to bridge this gap. Both within-host and between-host dynamics will contribute to defining landscapes of immunity and is a major focus of the lab in the context of immediate public health relevance. Broadly the Metcalf Lab blends basic ecological research with paramount applications. The underlying synthesis of demography and disease dynamics across scales ranging from within host to across and between countries is a distinguishing feature of the lab.