When someone is sick, the patient’s decision to take an antibiotic imposes costs on others—it helps bacteria evolve resistance faster. But it also gives free benefits to others: It may slow down the spread of infectious disease the same way that vaccinations do. Thus, antibiotics can create external costs as well as external benefits. In theory, these could cancel each other out, so that just the right amount of antibiotics is being used. But economists think that on balance, there is overuse of antibiotics, not underuse. Why?