What are the drivers of disease spread at the molecular and epidemiological level? The emergence and spread of pathogens is determined by pathogen transmission mechanisms and a breakdown in host-range restriction. Both are central to infectious disease and require understanding of (i) the interaction between molecules at the host-pathogen interface and (ii) the interactions between species and populations at an epidemiological level. We aim to identify the key parameters governing transmission at these different levels, applying quantitative analysis and evolutionary theory to understand how pathogens spread. Combining these different approaches will foster new ideas and strategies for disease containment. This article was published on 2025-03-12