Stephen Stearns, teaching Yale's Evolution and Medicine course, uses this short segment to unsettle a term doctors use every day: patient. Drawing on the 2015 textbook Evolutionary Medicine, he asks why individuals exposed to the same pathogen or environmental stressor end up with such different outcomes, some resistant, some susceptible, some asymptomatic carriers. Stearns frames this variation as an evolutionary problem rather than a clinical anomaly, pointing to genetic diversity in immune response and the selective pressures that maintain it across populations. The lecture sits within a larger series building toward a biological, rather than purely mechanical, model of disease, treating the patient as a product of evolutionary history and population-level variation rather than a generic case. It is a compact, lecture-style segment aimed at students already following the course, heavy on concepts and light on visual aids, functioning as a building block rather than a self-contained survey.