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Leprosy or Hansen's disease (HD), is a chronic disease caused by the bacteria Mycobacterium leprae and Mycobacterium lepromatosis.[1][2] Named after physician Gerhard Armauer Hansen, Leprosy is primarily a granulomatous disease of the peripheral nerves and mucosa of the upper respiratory tract; skin lesions are the primary external sign.[3] Left untreated, leprosy can be progressive, causing permanent damage to the skin, nerves, limbs and eyes. Contrary to folklore, leprosy does not cause body parts to fall off, although they can become numb and/or diseased as a result of infection.[4][5]

Although the mode of transmission of Hansen's disease remains uncertain, most investigators think that M. leprae is usually spread from person to person in respiratory droplets.[6] The minimum incubation period reported is as short as a few weeks and this is based on the very occasional occurrence of leprosy among young infants.[49] The maximum incubation period reported is as long as 30 years, or over, as observed among war veterans known to have been exposed for short periods in endemic areas but otherwise living in non-endemic areas. It is generally agreed that the average incubation period is between three and five years. Leprosy is now known to be neither sexually transmitted nor highly infectious after treatment. Approximately 95% of people are naturally immune[7] and sufferers are no longer infectious after as little as 2 weeks of treatment.

Leprosy has affected humanity for over 4,000 years,[8] and was well-recognized in the civilizations of ancient China, Egypt, and India.[9] DNA taken from the shrouded remains of a man discovered in a tomb next to the Old City of Jerusalem shows him to be the earliest human proven to have suffered from leprosy.[10] In 1995, the World Health Organization (WHO) estimated that between 2 and 3 million people were permanently disabled because of leprosy at that time.[11] In the past 20 years, 15 million people worldwide have been cured of leprosy.[12] Although the forced quarantine or segregation of patients is unnecessary in places where adequate treatments are available, many leper colonies still remain around the world in countries such as India (where there are still more than 1,000 leper colonies),[12] China,[13] Romania,[14] Egypt, Nepal, Somalia, Liberia, Vietnam,[15] and Japan.[16] Leprosy was once believed to be highly contagious and sexually transmitted, and was treated with mercury—all of which applied to syphilis which was first described in 1530. It is now thought that many early cases of leprosy could have been syphilis.[17]

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