The main features of TB as a community problem are well known. Its incidence is rare among people who lead an open air life and among those who live in small communities, but it increases in proportion to the degree of overcrowding. Among other factors contributing to the spread of the disease, mention may be made of malnutrition and undernutrition, unhygienic housing and environmental conditions and, certain occupations, particularly those associated with the inhalation of dust containing fine particles of silica.

No age, sex or race is exempted from TB. In countries where the disease has been prevalent for a long time, susceptibility to infection is highest among infants and a varying measure of protection becomes developed as the years go by, through small doses of infection being picked up by most individuals. For instance, only a small proportion of those who get infected, in Europe and America, develop the disease or die of it, while the majority acquire a considerable degree of protection from it. On the other hand, in communities exposed to TB for the first time, example, primitive races coming in contact with persons from the highly tuberculised countries, the disease occurs in a virulent form and the rate of its spread is rapid. In countries with a long history of TB infection, it is only among infants that conditions exist which approximate to those of the highly susceptible communities.