Stephen Stearns, teaching Yale's Evolution and Medicine course, asks a question that sounds simple but isn't: what exactly counts as a patient when a human body is host to trillions of microbial cells? Drawing on the textbook Evolutionary Medicine, Stearns walks through how the microbiota develops alongside the human organism from birth onward, shaping immune function, metabolism, and disease susceptibility in ways that classical medicine often ignored. He frames the human body as an ecosystem rather than a single organism, and argues that clinical practice needs to account for this composite biology if it wants to understand chronic disease, allergy, and infection accurately. The lecture is part of a numbered sequence (2.11) within the course, building on earlier sessions about evolutionary theory applied to medicine. It runs about fifteen minutes and functions as a focused unit rather than a standalone survey, aimed at students already grounded in the course's evolutionary framework.