Leprosy (Hansen's disease) is a chronic infectious disease caused by a bacterium Mycobacterium leprae affecting especially the skin and peripheral nerves, characterized by the formation of nodules and pale colored skin patches that enlarge and spread. Nerve damage can lead to muscle weakness, paralysis, wasting culminating in deformities, and loss of sensation in the skin patches. Leprosy's long incubation period makes it very difficult for doctors to determine when and where a person with leprosy got infected.