This introduction presents an overview of the key concepts covered in the subsequent chapters of this book. Tuberculosis has been seen to be an infectious disease having a variable period of incubation, and a course which may extend intermittently or continuously over many years. Tuberculosis is not only an infectious but also a chronic disease, which on the average probably extends over years and often escapes recognition during a large part of the time. Fallacy is almost inevitable in such a case if inferences as to causation are sought from individual groups of local statistics. It is well known that the phthisis death-rate is higher in populations inhabiting one room than in those inhabiting dwellings with two or more rooms; and is greatest in overcrowded dwellings of any given size. It will be found, however, that the course of tuberculosis has followed that of one element of sanitary environment, namely, the institutional segregation of tuberculous patients.