Matt Richtel uses the stories of four people to tell the incredible story of the human immune system, combining the latest science with interviews and anecdotes from the world’s leading researchers to reveal how the body marshals its forces to fight bacteria, viruses, parasites, and tumors. He also explains how, sometimes, this wondrous system that is our first line of defense can become a threat, attacking our organs and other systems. While modern life has freed us from many of the diseases that plagued humans in the past, it also puts unprecedented stress on the very system that keeps us healthy. The result has been an explosion of autoimmune disorders. Richtel details how immunologists are advancing medical breakthroughs, illuminates the close connection between the immune system and mental health, beauty, and weight, examines the latest findings about diet and nutrition on immunity, and offers us a glimpse of a future in which we lead longer, more comfortable lives. He also warns that we are weakening the system that protects us: rather than trusting the bodies’ natural defenses, many people turn to antibacterial soaps, immune boosters, and other products, resulting in the development of dangerous drug-resistant microbes that threaten health.