Stephen Stearns, teaching Yale's Evolution and Medicine course, explains why modern hygiene may be making immune systems misfire. He lays out the hygiene hypothesis, the idea that reduced childhood exposure to parasites, bacteria, and other microbes has left the immune system undertrained, and its refinement, the old friends hypothesis, which argues the immune system specifically needs contact with the microorganisms it coevolved with over millions of years rather than just any germs. Stearns connects this mismatch between ancestral and modern environments to the rise of allergies and autoimmune disorders in wealthy, sanitized societies. Drawing on the textbook Evolutionary Medicine, the segment is part of a longer lecture series applying evolutionary theory to human disease, framing hygiene-related illness as a predictable consequence of rapid environmental change outpacing immune adaptation.