Illustration of Mycobacterium leprae, a gram-positive bacteria which is the cause of leprosy (Hansen's disease). There are three forms of the disease, with lepromatous being the most severe, tuberculoid, and borderline leprosy. The disease is spread in humans by droplets of nasal mucus and a person is infectious only during the first stages of leprosy. Nerve damage usually occurs in limbs and facial areas, which may lead to skin damage, blindness, and disfigurement.