Leprosy, also known as Hansen's disease (HD), is a chronic infection caused by the bacteria Mycobacterium leprae and Mycobacterium lepromatosis.Initially, infections are without symptoms and typically remain this way from 5 to as long as 20 years.eprosy in Tahiti, circa 1895. Leprosy is primarily a granulomatous disease of the peripheral nerves and mucosa of the upper respiratory tract; skin lesions (light or dark patches) are the primary external sign. If untreated, leprosy can progress and cause permanent damage to the skin, nerves, limbs, and eyes.