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 a lack of ability to feel pain, 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 poor eyesight. Leprosy symptoms may begin within one year, but, for some people, symptoms may take 20 years or more to occur.

Leprosy is spread between people, although extensive contact is necessary. Leprosy has a low pathogenicity, and 95% of people who contract M. leprae do not develop the disease. Spread is thought to occur through a cough or contact with fluid from the nose of a person infected by leprosy. Genetic factors and immune function play a role in how easily a person catches the disease. Leprosy does 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. A number of 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 go to school and work. In the 1980s, there were 5.2 million cases globally but they went down to less than 0.2 million by 2020. Most new cases occur in 14 countries, with India accounting for more than half. In the 20 years from 1994 to 2014, 16 million people worldwide were cured of leprosy. About 200 cases per year are reported in the United States. Separating people affected by leprosy by placing them in leper colonies still occurs in some areas of India, China, the African continent, and Thailand.

Leprosy has affected humanity for thousands of years. The disease takes its name from the Greek word λέπρᾱ (léprā), from λεπῐ́ς (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. Some consider the word leper offensive, preferring the phrase "person affected with leprosy". Leprosy is classified as a neglected tropical disease. World Leprosy Day was started in 1954 to draw awareness to those affected by leprosy.