leprosy, or Hansen disease, Chronic disease of the skin, peripheral nerves, and mucous membranes of the nose, throat, and eyes, caused by the bacterium Mycobacterium leprae. In tuberculoid leprosy, cells of the immune system crowd into infected areas of the skin, forming hard nodules, or tubercles, that spread along nerve fibres. This type of reaction commonly leads to claw hand, gross deformity of the foot, and paralysis of muscles of the face, eye, and neck. In the lepromatous type, bacilli multiply freely in deep layers of the skin and spread widely through lymphatic channels and along nerve fibres, causing thickening and corrugation of the skin, raising soft nodules on the ears, nose, and cheeks, and sometimes destroying the septum of the nose and the palate. Leprosy has a long history. Until the 20th century, infected people were ostracized from society or at best segregated and cared for in isolated leper colonies. Today the disease is entirely curable through multidrug therapy, though tissue damage caused before drug treatment cannot be reversed. Some 600,000 new cases arise every year, mostly in Asia, Africa, Central and South America, and the Pacific Islands. About 60 percent of new cases occur in India.
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