Tuberculosis is an infectious disease caused by Mycobacterium tuberculosis that is spread person to person via airborne droplets. Subjects not previously exposed develop primary tuberculosis, characterized by the formation of a Ghon focus. Mycobacteria are carried by macrophages to lymph nodes, and hematogenous spread of disease may then occur. Most patients remain without symptoms and heal; however, in some patients, disease advances into progressive primary tuberculosis. In some patients, disease progresses after a latency period (progressive postprimary tuberculosis). Progressive tuberculosis is typically symptomatic and may be complicated by tuberculous bronchopneumonia, miliary tuberculosis, and tuberculous empyema.