Why do kids born in the summer get diagnosed more often with A.D.H.D.? How are marathons harmful for your health, even when you're not running? What do surgeons and salesmen have in common? Which annual event made people 30 percent more likely to get COVID-19? As a University of Chicago-trained economist and Harvard medical school professor and doctor, Anupam Jena is uniquely equipped to answer these questions. And as a critical care doctor at Massachusetts General who researches health care policy, Christopher Worsham confronts their impact on the hospital's sickest patients. In this singular work of science and medicine, Jena and Worsham show us how medicine really works, and its effect on all of us.