Stephen Stearns, in this segment of his Evolution and Medicine course, explains how evolutionary theory informs strategies for managing the human microbiota and the pathogens that colonize it. Drawing on the framework of the 2015 Sinauer textbook Evolutionary Medicine, Stearns walks through how bacteria and other microbes evolve under selective pressure from antibiotics, immune defenses, and competition within the body's microbial communities. He covers why pathogen virulence and resistance shift over time and what this means for treatment approaches that work with, rather than against, evolutionary dynamics. The lecture is short and focused, part of a longer numbered series, and assumes some prior grounding in evolutionary biology concepts covered earlier in the course. It is aimed at students trying to connect basic evolutionary principles to practical questions in medicine and public health.