Tuberculosis (TB) is an infectious disease caused by the bacteria Mycobacterium tuberculosis, which is spread by airborne droplets. The risk of infection depends upon duration of exposure, the instensity of the exposure, and the immune status of the person exposed. The immune system clears the bacteria immediately in more than 80% of people exposed. In a small proportion of people who have been infected, the bacteria are walled off and remain dormant but viable. This is called latent TB.