Tuberculosis (TB) is a disease caused by an organism called Mycobacterium tuberculosis, a rod-shaped bacillus. Occasionally the disease can also be caused by Mycobacterium bovis and Mycobacterium africanum. M tuberculosis is usually affecting the lungs in which case it is called pulmonary TB. Tuberculosis is mainly transmitted person-to-person by inhalation of infected droplet nuclei, which are expelled into the air when an untreated infectious pulmonary TB patient coughs or sneezes.