Leprosy (hanseniasis) is caused by chronic infection with Mycobacterium leprae (M. leprae). The disease involves primarily the superficial peripheral nerves and the skin, but almost any organ can be affected. The clinical features vary and are determined by the host's immune response to the infection. A distinction is made between multibacillary and paucibacillary forms of leprosy. The multibacillary forms are lepromatous, borderline-lepromatous and borderline-borderline leprosy; the paucibacillary forms are tuberculoid and borderline-tuberculoid leprosy. The clinical features and the histological picture depend on the patient's immune response. Because effective chemotherapy has become available, leprosy can now be cured, and frightening disabilities are therefore preventable.