It is readily apparent that certain disease organisms tend to locate themselves in definite regions and the question arises, Is this due to any specific relationship between organism and tissue or not? Diphtheria in man usually attacks the tonsils first, gonorrhea and syphilis the external genitals, tuberculosis the lung, “choleras” the small intestine, “dysenteries” the large intestine, influenza the lungs. In these cases the explanation is probably that the points attacked are the places where the organism is most commonly carried, with no specific relationship, since all of these organisms (Asiatic cholera excepted) also produce lesions in other parts of the body _when they reach them_. On the other hand, the virus of hydrophobia attacks nerve cells, leprosy frequently singles out nerves, glanders bacilli introduced into the abdominal cavity of a young male guinea-pig cause an inflammation of the testicle, malarial parasites and piroplasms attack the red blood corpuscles, etc. In fact, most _pathogenic protozoa_ are specific in their localization either in certain tissue cells or in the blood or lymph. In these cases there is apparently a real chemical relationship, as there is also between the _toxins_ of bacteria and certain tissue cells (tetanus toxin and nerve cells). Whether “chemotherapy” will ever profit from a knowledge of such chemical relationships remains to be developed. It appears that a search for these specific chemical substances with the object of combining poisons with them so that the organisms might in this way be destroyed, would be a profitable line of research. CHAPTER XXV. IMMUNITY. Immunity, as has already been stated, implies such a condition of the body that pathogenic organisms after they have been introduced are incapable of manifesting themselves, and are unable to cause disease. The word has come to have a more specific meaning than resistance in many instances, in other cases the terms are used synonymously. It is the opposite of susceptibility. The term must be understood always in a relative sense, since no animal is immune to all pathogenic organisms, and conceivably not entirely so to anyone, because there is no question that a sufficient number of bacteria of any kind might be injected into the circulation to kill an animal, even though it did it purely mechanically.