Stephen Stearns, teaching Yale's Evolution and Medicine course, asks what counts as a typical patient when disease resistance varies so much between individuals. Drawing on the textbook Evolutionary Medicine, he walks through why some people fight off infections or tolerate toxins that sicken others, framing this variation as an evolutionary problem rather than a statistical nuisance. The segment covers genetic diversity in immune response and how natural selection maintains a spread of susceptibility across a population instead of converging on one optimal defense. Stearns uses this to challenge the assumption built into much clinical training, that there is a single normal physiology against which patients deviate. Short and tightly focused, the lecture sets up the evolutionary logic that the rest of the course applies to specific diseases.