We can cure hundreds of ailments, but we understand remarkably little about why diseases exist at all. Why do we crave the very things that make us sick? Why, after thousands of generations, hasn't natural selection eliminated cancer, schizophrenia, and other scourges and evolved us into perfect human beings? Such questions are at the heart of the new discipline called Darwinian medicine, which applies the principles of evolutionary biology to the problems of medicine. The result of a unique collaboration between the chief architects of this new science - a groundbreaking Darwinian physician and one of the pioneers of modern evolutionary theory - Why We Get Sick offers a whole new set of scientific questions and suggests new ways of understanding illness. Finding evolutionary explanations for why we get sick - infectious agents that evolve faster than we do, environmental novelties, genes that are selected despite the fact that they cause disease, defenses, design compromises, evolutionary legacies - can help us uncover new, more effective methods of treatment. It can help resolve medical quandaries - for example, when is it best to let a fever run its course and when best to bring it down with medication? It offers a new view of disease that changes the relationship between our bodies and ourselves. Why We Get Sick takes the first major step toward reconceiving medicine as we approach the twenty-first century. Incorporating an evolutionary perspective into our understanding of illness will revolutionize the art and science of medicine and enable its practitioners to come close to achieving its ancient goals: To cure, sometimes. To help, often. To console, always.