The prevention of tuberculosis evidently must depend upon an accurate knowledge of the sources from which infection is derived. The possibility of infection by animal food-stuffs raises the large question of the intercommunicability of human and bovine tuberculosis. Tuberculosis from lower animals is only likely to be conveyed to man to any considerable extent by the ingestion of infected foods, especially milk. The subcutaneous injection of tubercle bacilli in experimental animals produces tuberculosis which, following the lymphatic tracts, may soon become general. Lupus, a disease eventually causing a disfiguring ulceration of the skin, is a local form of tuberculous infection. This chapter also considers certain modes of infection, less important than the inhalation of infective dust or spray, but conveniently disposed of at this stage. These methods consist in inoculation with tubercle bacilli, infection by kissing or by other means of conveying infected saliva, and infection by contaminated hands or by flies.