Stephen Stearns, teaching Yale's Evolution and Medicine course, covers how pathogens evolve mechanisms to escape host immune defenses. Drawing on the textbook Evolutionary Medicine, he walks through strategies such as antigenic variation, rapid mutation rates, and exploitation of host signaling pathways that let bacteria, viruses, and parasites outpace immune recognition. The segment fits within a larger unit on pathogen evolution, connecting molecular tricks like surface protein switching to broader evolutionary logic: any defense a host mounts creates selection pressure for a pathogen countermeasure. Stearns keeps the focus on mechanism, citing specific examples of immune evasion drawn from clinical and laboratory research. Though brief, the lecture distills a dense evolutionary arms race into a clear, structured explanation aimed at students studying how disease agents persist despite immune systems built to eliminate them.