TB infections are usually followed by an immune response and latency after exposure. Most persons infected with M. tuberculosis do not experience clinical illness and are non-infectious. (Recall that this phase is referred to as LTBI.) About 5-10% of persons who are infected and are not treated will progress to develop active TB during their lifetime. The risk for progression is highest during the first several years after infection and in persons with underlying health conditions.
Infection occurs when a susceptible person inhales infectious droplet nuclei containing Mycobacterium tuberculosis, and the organism reaches the alveoli of the lungs. TB has a very low infectious dose, and the minimal infectious inoculum may be as low as 1-10 viable organisms.
About 2-12 weeks after exposure and infection, the immune system in healthy individuals will limit the multiplication of additional TB bacteria, and an immunological test can positively detect the presence of the organism.
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 persons infected with HIV.
