Also known as Hansen's disease, this is a chronic bacterial infection caused by Mycobacterium leprae and affecting the skin, mucous membranes, and nerves. Infection is now almost confined to South East Asia, localised areas in Africa and a small number in Brazil. There are two distinct clinical forms: tuberculoid and lepromatous. The former usually takes a benign course and frequently burns out, whereas the latter is relentlessly progressive. Nasal secretions (especially in lepromatous disease) constitute the main source of infection but living in close proximity to an infected individual seems necessary for someone to contract the disease. M. leprae can also be transmitted in breast milk from an infected mother.

The incubation period is 3–5 years or longer. The major clinical manifestations involve skin and nerves: the former range from depigmented, often anaesthetic areas, to massive nodules; nerve involvement ranges from localised nerve swelling(s) to extensive areas of anaesthesia. Advanced nerve destruction gives rise to severe deformities, including foot-drop, wrist-drop, claw-foot, extensive ulceration of the extremities with loss of fingers and toes, and bone changes. Eye involvement can produce blindness. Laryngeal lesions produce hoarseness and more serious sequelae. Skin-smears, histological features and the lepromin skin-test help to confirm the diagnosis and enable the form of disease to be graded.