Leprosy, also known as Hansen's disease (HD), is a long-term infection by the bacteria Mycobacterium leprae or Mycobacterium lepromatosis. Infection can lead to damage of the nerves, respiratory tract, skin, and eyes. This nerve damage may result in the loss of nociception, which can lead to the loss of parts of a person's extremities from repeated injuries or infection through unnoticed wounds. An infected person may also experience muscle weakness and loss of eyesight. Leprosy symptoms may begin within one year or take 20 years or more.

Leprosy is spread between people, although extensive contact is necessary. Leprosy has a low pathogenicity, and 95% of people who contract or who are exposed to M. leprae do not develop the disease. Spread is likely through a cough or contact with fluid from the nose of a person infected by leprosy. Genetic factors and baseline immune function play a role in how easily a person catches the disease. Leprosy is not spread during pregnancy to the unborn child or through sexual contact. Leprosy occurs more commonly among people living in poverty. There are two main types of the disease—paucibacillary and multibacillary, which differ in the number of bacteria present. A person with paucibacillary disease has five or fewer poorly pigmented, numb skin patches, while a person with multibacillary disease has more than five skin patches. The diagnosis is confirmed by finding acid-fast bacilli in a biopsy of the skin.

Leprosy is curable with multidrug therapy. Treatment of paucibacillary leprosy is with the medications dapsone, rifampicin, and clofazimine for six months. Treatment for multibacillary leprosy uses the same medications for 12 months. Several other antibiotics may also be used. These treatments are provided free of charge by the World Health Organization.

Leprosy is not highly contagious. People with leprosy can live with their families and attend school and work. In the 1980s, there were 5.2 million cases globally, but by 2020 this decreased to fewer than 200,000. Most new cases occur in one of 14 countries, with India accounting for more than half of all new cases. In the 20 years from 1994 to 2014, 16 million people worldwide were cured of leprosy. Separating people affected by leprosy by placing them in leper colonies is not supported by evidence but still occurs in some areas of India, China, Japan, Africa, and Thailand.

Leprosy has affected humanity for thousands of years. The disease takes its name from the Greek word λέπρα (lépra), from λεπίς (lepís; 'scale'), while the term "Hansen's disease" is named after the Norwegian physician Gerhard Armauer Hansen. Leprosy has historically been associated with social stigma, which continues to be a barrier to self-reporting and early treatment. Leprosy is classified as a neglected tropical disease. World Leprosy Day was started in 1954 to draw awareness to those affected by leprosy. The study of leprosy and its treatment is known as leprology.

Common symptoms present in the different types of leprosy include a rhinorrhea; dry scalp; vision problems; skin lesions; muscle weakness; reddish skin; smooth, shiny, diffuse thickening of facial skin, ear, and hand tissues; loss of sensation in fingers and toes; thickening of peripheral nerves; a flat nose from the destruction of nasal cartilage; and changes in phonation and other aspects of speech production. In addition, atrophy of the testes and erectile dysfunction may occur.

Leprosy onset varies between individuals. The average incubation period is five years, but the infected may begin to notice symptoms within the first year or up to 20 years after infection. Oftentimes, the first noticeable sign of leprosy is the development of pale or pink-coloured patches of skin that may be insensitive to temperature or pain. Patches of discolored skin are sometimes accompanied or preceded by nerve problems, including numbness or tenderness in the hands or feet. Secondary infections (bacterial or viral infections consequent to the primary infection) can result in tissue loss, causing fingers and toes to become shortened and deformed as cartilage is absorbed into the body. Baseline immune function drives at least some parts of pathogenesis variability.

Approximately 30% of individuals affected by leprosy experience nerve damage. The nerve damage sustained is reversible when treated early, but becomes permanent when appropriate treatment is delayed by several months. Damage to nerves may cause loss of muscle function, leading to paralysis. It may also lead to sensation abnormalities or numbness, leading to additional infections, ulcerations, and joint deformities.