Leprosy (from the Greek lepi (λέπι), meaning scales on a fish), or Hansen's disease, is a chronic disease caused by the bacteria Mycobacterium leprae and Mycobacterium lepromatosis. Leprosy is primarily a granulomatous disease of the peripheral nerves and mucosa of the upper respiratory tract; skin lesions are the primary external symptom. Left untreated, leprosy can be progressive, causing permanent damage to the skin, nerves, limbs and eyes. Contrary to popular belief, leprosy does not actually cause body parts to simply fall off
Mycobacterium leprae and Mycobacterium lepromatosis are the causative agents of leprosy. M. lepromatosis is only the causitive agent in diffuse lepromatous leprosy, which can be lethal. An intracellular, acid-fast bacterium, M. leprae is aerobic and rod-shaped, and is surrounded by the waxy cell membrane coating characteristic of Mycobacterium species. Due to extensive loss of genes necessary for independent growth, M. leprae and M. lepromatosis are unculturable in the laboratory, a factor which leads to difficulty in definitively identifying the organism under a strict interpretation of Koch's postulates. The use of non-culture-based techniques such as molecular genetics has allowed for alternative establishment of causation.
A single dose of rifampicin is able to reduce the rate of leprosy in contacts by 57% to 75%. BCG is able to offer a variable amount of protection against leprosy as well as against tuberculosis.