This book argues that religion has emerged over evolutionary time as a strategy for managing the transmission, contraction, and eradication of infectious disease. From purity and pollution codes to blood sacrifices and irrational beliefs, the book shows how religion supports not only the physiological immune system, but the behavioral and psychological immune systems as well. The book also addresses those moments when it appears that religion becomes maladaptive, that is, when religion causes “autoimmune problems,” such as celibacy and anti-vaccination. Engaging material ranging from evolutionary and social psychology to human behavioral ecology, biological anthropology, Darwinian medicine, and religious studies, the book proposes that in order to understand the human animal’s enduring fascination with religion, one must take into account the enduring need to manage infectious disease.