Evolutionary biologist Stephen Stearns, teaching Yale's Evolution and Medicine course, tackles a deceptively simple question: what actually counts as a disease. He walks through the idea that organisms sit somewhere on a spectrum from vulnerable to robust, and that whether a condition counts as pathological depends on how much it degrades an organism's ability to function and survive. Stearns draws on the course's companion textbook, Evolutionary Medicine, to frame disease not as a fixed category but as a consequence of trade-offs built into evolved bodies, where defenses that work in one context become liabilities in another. The lecture is short and pointed, functioning as one building block in a longer sequence rather than a stand-alone survey, and it sets up definitions the course will keep returning to.