A chronic infectious disease caused by Mycobacterium leprae that primarily affects the skin and peripheral nerves and, to a lesser extent, involves the eyes and mucous membranes. Leprosy, or Hansen's disease, has been known for more than 2000 years. It afflicts at least 3 million people worldwide and is most common in developing countries. There are about 200 new cases yearly in the United States, 85% of them among aliens. The pathogen, M. leprae, has never been cultured in artificial media, but it can be grown in the mouse (in the footpad) and the nine-banded armadillo, among other animals. About 2–3% of armadillos in the wild in the southern United States harbor the infection. Its epidemiology is not fully understood, but transmission probably takes place by the respiratory route. The bacillus is very slow-growing, and the incubation period is usually 3–5 years. Less than 5% of any population is susceptible, and these individuals have a deficient cell-mediated immune response specifically to M. leprae, which may be genetic in origin. Epidemics have occurred, but are rare. Historically, considerable stigma has been attached to the disease and remains a real, though diminishing, problem.