The natural history of TB infection is usually followed by an immune response and latency after exposure. In about 5-10% of cases, the latent period progresses to an active infection.

The organism that causes TB infection is Mycobacterium tuberculosis. This organism is pictured in the photograph to the right as observed when stained with acridine orange stain.

Infection occurs when a susceptible person inhales droplet nuclei containing Mycobacterium tuberculosis and the organism reaches the alveoli of the lungs.

About 2-12 weeks after infection, the immune system limits multiplication of additional bacteria and the immunological test becomes positive.

Latent tuberculosis infection (LTBI) is the stage when the viable organism remains in the body, and the patient has no symptoms and is non-infectious.

Most infected persons do not experience clinical illness and are noninfectious. About 5-10% of persons infected with Mycobacterium tuberculosis who are not treated will develop TB during their lifetime. The risk for progression is highest during the first several years after infection.

TB infects the lungs most often; however, it can infect almost any organ in the body, including bones and joints.