Around Christmas of 1882, while peering through a microscope at starfish larvae in which he had inserted tiny thorns, Russian zoologist Elie Metchnikoff had a brilliant insight: what if the mobile cells he saw gathering around the thorns were nothing but a healing force in action? Metchnikoff's daring theory of immunity--that voracious cells he called phagocytes formed the first line of defense against invading bacteria--would eventually earn the scientist a Nobel Prize, shared with his archrival, as well as the unofficial moniker "Father of Natural Immunity." But first he had to win over skeptics, especially those who called his theory "an oriental fairy tale." Using previously inaccessible archival materials, author Luba Vikhanski chronicles Metchnikoff's remarkable life and discoveries in the first modern biography of this hero of medicine. Metchnikoff was a towering figure in the scientific community of the early twentieth century, a tireless humanitarian who, while working at the Pasteur Institute in Paris, also strived to curb the spread of cholera, syphilis, and other deadly diseases. In his later years, he startled the world with controversial theories on longevity, launching a global craze for yogurt, and pioneered research into gut microbes and aging. Though Metchnikoff was largely forgotten for nearly a hundred years, Vikhanski documents a remarkable revival of interest in his ideas on immunity and on the gut flora in the science of the twenty-first century.