The history of tuberculosis as a global epidemic.

TB should be thought of as a slowly progressing worldwide epidemic. Initially it was a disease of lower mammals, and the etiologic agent probably preceded the development of man on earth. It became an uncommon endemic disease in man about the time man began to settle in villages and develop agriculture. Crowding in European cities, and later the industrial revolution in Europe, provided the necessary environmental conditions for the endemic disease to become epidemic. For the next 400 years, the disease was spread by European empire-building and colonization. It came late to sub-Saharan Africa and to the Pacific Islands, and still later to the highlands of New Guinea. The epidemic gradually wanes within a large population group as resistant individuals survive and reproduce. This natural resistance is reflected in the ability of the macrophage to control intracellular growth of the organism. The resistant host shows a chronic infection primarily affecting the lungs, whereas the highly susceptible host shows a rapidly fatal illness with generalized spread of disease to many organs. Survivors of the initial infection then show another type of resistance to reinfection that is based on sensitized T cells. When this system is only partially successful, the host becomes infectious and capable of spreading the infection widely. The study of the epidemiology of TB and the evaluation of various public health measures to prevent or contain the disease requires that the investigator have an understanding of the nature and duration of the TB epidemic in the particular population under study. This factor is a much greater determinant of the course of an epidemic than any public health measure that man can institute, just as the currents in a river can have a more powerful effect on the course of a canoe than the most vigorous paddler.