Stephen Stearns opens a segment on the evolution of virulence in pathogens, part of his Yale course Evolution and Medicine, built around the 2015 Sinauer textbook Evolutionary Medicine. He lays out why virulence is not a fixed trait of a pathogen but an evolved outcome shaped by transmission mode, host response, and the trade-offs a pathogen faces between replicating fast inside a host and surviving long enough to spread to the next one. The lecture sets up the core logic evolutionary biologists use to explain why some infections are mild and others lethal, framing virulence as a variable under natural selection rather than an accident of a germ's nature. Aimed at students already following the course, it is a short, focused segment rather than a stand-alone survey, laying groundwork for the fuller argument that follows in later parts of the same lecture.