This chapter deals with tuberculosis from the point of view of preventive medicine and public health. It considers the sanatorium treatment of consumptives, this will be chiefly considered as a means of preventing others from becoming consumptive. Pulmonary tuberculosis is caused by the invasion of the lungs by the tubercle bacillus. The tubercle bacilli have entered the body and the leucocytes have failed to kill them. The earliest and most characteristic lesion produced is the grey tubercle. Leucocytes are killed by the bacterial toxins, and their dead bodies accumulate as pus. Sometimes profuse haemoptysis is the earliest symptom recognised, and it is often the first symptom which induces a patient to consult a doctor. From the description of the lesions found in fatal cases of phthisis the symptoms of the fully established disease may be gathered. The great majority of cases belong to the chronic variety.