Tuberculosis is a chronic infectious disease caused by Mycobacterium tuberculosis and is characterized by immunity against infection, including innate and acquired immunity. Innate immunity is the natural resistance against TB, consisting of surface and internal defense systems, and is non-specific. For example, the body surface barrier; the ciliated epithelium of the respiratory tract; alveolar macrophages; phagocytosis by neutrophils, monocytes, and macrophages; and bactericidal action of lysozyme. According to studies on the association of HLA with tuberculosis, certain loci were found to be more prevalent in tuberculosis patients, and the incidence of tuberculosis in both identical twins was much higher than in dizygotic twins. These facts suggest that susceptibility to TB (i.e., low natural resistance) is inherited and that this is a factor in the occurrence of TB disease. On the other hand, the vast majority of people infected with Mycobacterium tuberculosis can remain free of the disease for the rest of their lives, with non-specific immunity playing a role in addition to the influence of social factors. Acquired immunity is the specific immunity of the organism against the pathogen after stimulation by the immunogen of Mycobacterium tuberculosis, and cellular immunity plays a decisive role, while humoral immunity has no important influence. The organism may have some acquired immunity after natural infection with Mycobacterium tuberculosis, which was confirmed by the experiments of R. Koch. He injected a certain amount of tuberculosis bacilli into uninfected guinea pigs and found that the local reaction to the injection was slow but strong, even resulting in systemic progressive tuberculosis and death, while the local reaction to the injection of tuberculosis bacilli into infected guinea pigs was fast and faded quickly, without spreading to the whole body, which is the “Koch’s phenomenon. However, natural infection has both the risk of immediate onset of disease and, on the other hand, the presence of a primary lesion in the body after infection, which may, under certain adverse conditions, lead to active tuberculosis at any time by “endogenous rekindling”. Therefore, natural infection has more disadvantages (morbidity) than advantages (weak acquired immunity), while acquired immunity by BCG injection has more advantages (protection) than disadvantages (BCG side effects, etc.).