Tuberculosis (TB) is a communicable disease caused by bacteria of the "tuberculosis complex group" (mainly Mycobacterium tuberculosis [MTB] and rarely M. bovis, M. africanum and M. microti). The infection is transmitted from one person to another through invisible droplet nuclei which are generated when someone with active TB of the lungs or larynx coughs, sneezes, spits, laughs or talks. Active TB may also occur in sites outside the airways but transmission from these sites ( e.g., discharging wounds or abscesses) is uncommon. Transmission is relatively inefficient (in comparison to highly contagious diseases such as measles and chickenpox) and depends on the infectivity of the source case, as well as the amount of time spent in contact with others and the environment in which contact occurs. Conditions such as overcrowding in poorly ventilated enclosed spaces that are not exposed to sunlight (which kills MTB bacilli) greatly enhance the risk of
transmission.