It is spread almost exclusively by the respiratory route, by coughing, singing or talking. As aerosolized MTb bacilli enter the lungs, they multiply, and are transported by alveolar macrophages to the regional lymph nodes.
Hematologic dissemination occurs from the lymph nodes.
Exposure to MTb can result in latent infection, which can progress rapidly to active disease in individuals whose cell-mediated immunity becomes compromised by aging, malnutrition, chronic disease, or HIV infection. HIV infection is the most potent cofactor for progression from latent infection to active disease. In the developing world, Tb is the leading cause of AIDS-related death due to opportunistic infection. Tuberculosis is preventable and generally curable, but can be fatal if left untreated. Patients with HIV infection can develop active Tb from either latent infection activated by immune suppression, or from primary exposure to the organism.