Leprosy has affected humanity for thousands of years. The disease takes its name from the Greek word (), from (; 'scale'), while the term "Hansen's disease" is named after the Norwegian physician Gerhard Armauer Hansen. Leprosy has historically been associated with social stigma, which continues to be a barrier to self-reporting and early treatment. Some consider the word leper offensive, preferring the phrase "person affected with leprosy". Leprosy is classified as a neglected tropical disease. World Leprosy Day was started in 1954 to draw awareness to those affected by leprosy. Signs and symptoms Common symptoms present in the different types of leprosy include a runny nose; dry scalp; eye problems; skin lesions; muscle weakness; reddish skin; smooth, shiny, diffuse thickening of facial skin, ear, and hand; loss of sensation in fingers and toes; thickening of peripheral nerves; a flat nose from destruction of nasal cartilage; and changes in phonation and other aspects of speech production. In addition, atrophy of the testes and impotence may occur. Leprosy can affect people in different ways. The average incubation period is five years. People may begin to notice symptoms within the first year or up to 20 years after infection. The first noticeable sign of leprosy is often the development of pale or pink coloured patches of skin that may be insensitive to temperature or pain. Patches of discolored skin are sometimes accompanied or preceded by nerve problems including numbness or tenderness in the hands or feet. Secondary infections (additional bacterial or viral infections) can result in tissue loss, causing fingers and toes to become shortened and deformed, as cartilage is absorbed into the body. A person's immune response differs depending on the form of leprosy. Approximately 30% of people affected with leprosy experience nerve damage. The nerve damage sustained is reversible when treated early, but becomes permanent when appropriate treatment is delayed by several months. Damage to nerves may cause loss of muscle function, leading to paralysis. It may also lead to sensation abnormalities or numbness, which may lead to additional infections, ulcerations, and joint deformities. Cause M. leprae and M. lepromatosis M. leprae and M. lepromatosis are the mycobacteria that cause leprosy. M. lepromatosis is a relatively newly identified mycobacterium isolated from a fatal case of diffuse lepromatous leprosy in 2008. M. lepromatosis is indistinguishable clinically from M. leprae. M. leprae is an intracellular, acid-fast bacterium that is aerobic and rod-shaped. M.