Tuberculosis is undoubtedly caused most often by domestic infection. Koch says that tuberculosis has been frankly and justly called a dwelling disease; while Biermer goes further and describes it as essentially a bedroom disease. In treating of domestic infection it is necessary to distinguish between indirect and direct infection. The experimental results of Comet and others show that tubercle bacilli are present, but only in the immediate environment of consumptives. Although it is in the highest degree desirable that such a house should be efficiently cleansed and disinfected, it is unlikely to form a large element in the production of phthisis by domestic infection. There is abundant statistical evidence of the close association between overcrowding and excessive mortality from phthisis. Observations of supposed infection between married couples or its absence are trustworthy only if they accurately state the length of the married life of the couples under observation, and the subsequent history through life of the surviving partner.