Systematic account of biological defenses against infection that emphasizes the role of cellular phagocytosis alongside soluble serum factors. It synthesizes experimental results, comparative pathology, and theoretical argument to explain how organisms resist or succumb to microbes, discusses mechanisms of acquired resistance and vaccination, and evaluates rival explanations. Practical consequences for preventing and treating contagious diseases are explored, and a concise historical survey at the end retraces key experiments and controversies that shaped contemporary concepts of immunity.