Leprosy, also known as Hansen's disease (HD), is a chronic infection caused by the bacteria Mycobacterium leprae and Mycobacterium lepromatosis. Named after physician Gerhard Armauer Hansen, leprosy is primarily a granulomatous disease of the peripheral nerves and mucosa of the upper respiratory tract; skin lesions are the primary external sign. Left untreated, leprosy can be progressive, causing permanent damage to the skin, nerves, limbs and eyes. Contrary to folklore, leprosy does not cause body parts to fall off, although they can become numb or diseased as a result of secondary infections; these occur as a result of the body's defenses being compromised by the primary disease. Secondary infections, in turn, can result in tissue loss causing fingers and toes to become shortened and deformed, as cartilage is absorbed into the body.

Treatment for multibacillary leprosy consists of rifampicin, dapsone, and clofazimine taken over 12 months. Single dose MDT for single lesion leprosy consists of rifampicin, ofloxacin, and minocycline. The move toward single-dose treatment strategies has reduced the rates of disease in some regions. World Leprosy Day was created to draw awareness to those affected by leprosy.

In 1995, the World Health Organization (WHO) estimated that between 2 and 3 million people were permanently disabled because of leprosy at that time. In the past 20 years, 15 million people worldwide have been cured of leprosy.

Leprosy has affected humanity for over 4,000 years, and was recognized in the civilizations of ancient China, Egypt and India. Although the forced quarantine or segregation of patients is unnecessary in places where adequate treatments are available, many leper colonies still remain around the world in countries such as India (where there are still more than 1,000 leper colonies), China, Romania, Egypt, Nepal, Somalia, Liberia, Vietnam, and Japan. Leprosy was once believed to be highly contagious and was treated with mercury — all of which applied to syphilis, which was first described in 1530. It is possible that many early cases thought to be leprosy could have actually been syphilis. The age-old social stigma associated with the advanced form of leprosy lingers in many areas, and remains a major obstacle to self-reporting and early treatment. Effective treatment first appeared in the late 1940s. Resistance has developed to initial treatment. It was not until the introduction of multidrug therapy (MDT) in the early 1980s that the disease could be diagnosed and treated successfully within the community.