Stress. It's always been there to save our lives. It's what made us run from predators and enabled us to take down prey. But today, humans are turning on that same stress response to deal with 30-year mortgages, difficult bosses, teenagers and traffic jams. Some of us are wallowing in corrosive hormones; for the first time, scientists can reveal just how measurable and dangerous that exposure can be. MacArthur "Genius Grant" recipient and Stanford University neurobiologist Robert Sapolsky and National Geographic search for answers to why stress seems to be killing us.