Causes leprosy (Hansen's disease) No culture in vitro, on footpads of mice and armadillos Acid fast rods, obligate intracellular; optimal growth is at cooler parts of the body (skin, mucous membranes, and peripheral nerves) Spread by droplets, prolonged exposure Incubation period 2-10 years, 10% develop disease Tuberculoid leprosy and lepromatous leprosy