Mutilated hands of an elderly man with leprosy. Leprosy (Hansen's disease) is a chronic infectious disease caused by Mycobacterium leprae, an acid-fast, rod-shaped bacillus, which has struck fear into human beings for thousands of years. It affects the skin and mucous membranes, and irreversibly damages nerves. The disease is treated with antibacterial drugs According to the World Health Organisation there were 224,717 registered cases of leprosy in 109 countries and territories in 2007