The most striking phenomenon in an attack of an infectious disease is its effect in preventing recurrence after recovery. In 1884 Metschnikoff observed that in water fleas the wandering cells of the body surrounded and gradually absorbed into their substance particles of pigment swallowed by the flea. On these and similar experiments he based his theory of phagocytosis, as responsible for securing immunity against infection. It soon became evident that, while body cells were active in the production of immunity, and much controversy arose between those who regarded the cells of the body and those who thought that the blood serum was responsible for immunity. While investigating the general problem of immunity there had been discovered a method of serum diagnosis, by which several infectious diseases can be differentiated. Various attempts have been made to immunise against catarrhs, with doubtful and erratic success. The whole group of catarrhal infections forms a wide barren patch in the field of preventive medicine.