With employers offering free flu shots and pharmacies expanding into one-stop shops to prevent everything from shingles to tetanus, vaccines are ubiquitous in contemporary life. The past fifty years have witnessed an enormous upsurge in vaccines and immunization in the United States: American children now receive more vaccines than any previous generation, and laws requiring their immunization against a litany of diseases are standard. Yet, while vaccination rates have soared and cases of preventable infections have plummeted, an increasingly vocal cross section of Americans have questioned...