Leprosy: An infectious disease of the skin, nervous system, and mucous membranes that is caused by the bacteria Mycobacterium leprae. Leprosy is transmitted via person-to-person contact. For thousands of years leprosy was one of the world's most feared communicable diseases because the nerve and skin damage often led to terrible disfigurement and disability. Today leprosy can be cured, particularly if treatment is begun early. Antibiotic therapy is the mainstay of treatment. Surgery can be performed to reconstruct damaged faces and limbs. Also known as Hansen's disease.