Figure 1.

Progression of human M. tuberculosis infections (1) Human tuberculosis infections are transmitted through the inhalation of contaminated aerosols. (2) Primary infections are established in the lungs, at a single site that develops into a primary granuloma. These initial sites of infection include involvement of the surrounding lymph nodes, forming a Ghon's complex. (3) Bacteria disseminate out of the lungs and into the lymphatic system, most likely entering the circulatory system through the entry of the thoracic duct into the subclavian vein. (4) Hematogenous reseeding of the lungs results in secondary granulomas located in the apical regions of the lungs and/or the extrapulmonary organs (5).