Stephen Stearns, professor of ecology and evolutionary biology at Yale, explains how pathogens evolve resistance to the drugs and defenses used against them. Drawing on the textbook Evolutionary Medicine, the lecture covers the mechanisms by which bacteria and other pathogens acquire resistance mutations, how selection pressure from antibiotics accelerates the spread of resistant strains, and why rapid pathogen generation times make resistance such a persistent clinical problem. Stearns frames the material within an evolutionary biology approach to medicine, connecting basic principles of natural selection to practical questions about treating infectious disease. Part of Stearns' Evolution and Medicine course, the segment is concise and focused, aimed at students who already have some grounding in evolutionary theory and want to see it applied directly to a pressing public health issue.