It is now a generally accepted opinion that tuberculous infection occurs, as a rule, through inhalation of tubercle bacilli. In whatever way the bacilli are inhaled, they finally lodge in an alveolus. There they multiply quickly and produce irritation of the alveolar epithelium. The anatomic result is a pneumonic inflammatory process. In occult cases the pneumonia affects only a few alveoli; in serious cases it may gradually involve the greater part of an entire lobe. Histologically regarded, the pneumonia does not have the appearance that is characteristic of tuberculosis. It can be recognized only by the presence of a great number of tubercle bacilli. This tuberculous pneumonia is called the primary focus; it is the first anatomic lesion in the organism produced by the infection.
Soon after the lungs have been infected, the tubercle bacilli spread from the alveoli along the lymphatics to the regional glands in the hilus. On