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.
Infection occurs when a susceptible person inhales droplet nuclei containing Mycobacterium tuberculosis and the organism reaches the alveoli of the lungs. The minimal infectious inoculum may be as low as one viable organism.
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; the individual has no symptoms and is noninfectious.
Most persons infected with M. tuberculosis do not experience clinical illness and are noninfectious. About 5-10% of persons who are infected and are not treated will develop active TB during their lifetime. The risk for progression is highest during the first several years after infection.
Most often, M. tuberculosis infects the lungs. However, it can infect almost any organ in the body, including bones and joints. Tuberculosis meningitis is a TB infection that occurs outside the lungs with devastating consequences, most often in young children and patients with AIDS.