This chapter discusses the constant correspondence between the variations in the prevalence of tuberculosis and in any element of sanitary environment consists in the relation to tuberculosis of the institutional segregation of patients. Whether for good or harm, the segregation of infective patients is likely to influence the spread of tuberculosis. The vast majority of pathologists and hygienists are agreed that the chief source of infection in human tuberculosis is the tuberculous human patient. It is evident that institutional segregation is different qualitatively from domestic segregation. It remains to see how far the institutional segregation of infective patients which is secured in institutions in general has in actual fact served to control the spread of the disease. A brief statement of the history of typhus fever in Ireland and of leprosy in Norway throws some side-light on the influence of segregation in two other infectious diseases, one very acute and the other very chronic in its course.