- This article is about the infectious disease also known as Hansen's disease. For the malady found in the Hebrew Bible, see the article Tzaraas.
Leprosy, sometimes known as Hansen's disease, is an infectious disease caused by infection by Mycobacterium leprae, an aerobic, acid fast, rod-shaped mycobacterium. The modern name of the disease comes from the discoverer of Mycobacterium leprae, Gerhard Armauer Hansen. Sufferers from Hansen's disease have generally been called lepers, although this term is falling into disuse both from the diminishing number of leprosy patients and from pressure to avoid the demeaning connotations of the term. Also, this term can lead to public misunderstanding because the terms leprosy and leper are used in the Bible to describe a wide range of incurable skin conditions.
Leprosy used to be incurable and severely disfiguring. Lepers were shunned and sequestered in leper colonies. Today, leprosy is easily curable by multidrug antibiotic therapy. The main challenges for Hansen's disease elimination efforts are to reach populations that have not yet received multidrug therapy services, improve detection of the disease, and provide patients with high-quality services and affordable drugs.
Other than humans, the only animal known to be susceptible to leprosy is the armadillo.
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Hansen's disease has been recognized as a problem since the beginning of recorded history. It has been reported as early as 1350 BC in Egypt, making it the oldest disease known according to Guinness World Records. Lepers have frequently lived on the edge of society, and the disease was often believed to have been caused by a divine (or demonic) curse or punishment. However, in the Middle Ages it was believed that lepers are cursed by humans, but loved by God.
The Bible contains many references to "leprosy", which do not necessarily concern Hansen's disease. These words seem to have been used to cover a number of skin conditions of different etiology and severity. Under ancient Israelite law, the priests were required to be able to diagnose leprosy. The Israelites also used quarantine to prevent its spread.
In the Middle Ages, it was believed that leprosy is highly contagious and could be spread by the glance of a leper or an unseen leper standing upwind of healthy people. Nowadays, it is known that leprosy is much less contagious.
The disease is caused by a mycobacterium which multiplies very slowly and mainly affects the skin, nerves, and mucous membranes. The organism has never been grown in bacteriologic media or cell culture, but has been grown in mouse foot pads and more recently in nine-banded armadilloes. It is related to M. tuberculosis, the mycobacterium that causes tuberculosis. The difficulty in culturing the organism appears to be due to the fact that the organism is an obligate intra-cellular parasite that lacks many necessary genes for independent survival. This loss of genes is apparently also the reason for the extremely slow replication rate.
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. What is known is that the transmission rate is very low. In addition, it appears that a majority of the population is naturally immune. Also, contrary to popular belief, Hansen's disease does not cause rotting of the flesh; however, due to nerve damage, extremities may become numb which may lead to minor infected wounds being unnoticed until damage is permanent.
This chronic infectious disease usually affects the skin and peripheral nerves but has a wide range of possible clinical manifestations. Patients are classified as having paucibacillary or multibacillary Hansen's disease. Paucibacillary Hansen's disease is milder and characterized by one or more hypopigmented skin macules. Multibacillary Hansen's disease is associated with symmetric skin lesions, nodules, plaques, thickened dermis, and frequent involvement of the nasal mucosa resulting in nasal congestion and epistaxis (nose bleeds).
In 1999, the world incidence of Hansen's disease was estimated to be 640,000; and in 2000, 738,284 cases were identified. In 1999, 108 cases occurred in the United States. In 2000, the World Health Organization (WHO) listed 91 countries in which Hansen's disease is endemic, with India, Myanmar, and Nepal having 70% of cases. In 2002, 763,917 new cases were detected worldwide, and in that year the WHO listed Brazil, Madagascar, Mozambique, Tanzania and Nepal as having 90% of Hansen's disease cases.
Worldwide, one to two million people are permanently disabled because of Hansen's disease. However, persons receiving antibiotic treatment or having completed treatment are considered free of active infection.
Hansen's disease is one of the infectious diseases tracked passively by the Centers for Disease Control and Prevention. Its prevalence in the United States has remained low and relatively stable. There are decreasing numbers of cases worldwide, though pockets of high prevalence continue in certain areas such as Brazil, South Asia (India, Nepal), some parts of Africa (Tanzania, Madagascar, Mozambique) and the western Pacific.
Those having close contacts with patients with untreated, active, predominantly multibacillary disease, and persons living in countries with highly endemic disease are at risk of contracting the disease. Recent research suggests that there is genetic variation in susceptibility. The region of DNA responsible for this variability is also involved in Parkinson's disease, giving rise to current speculation that the two disorders may be linked in some way at the biochemical level.
There are still a few leper colonies around the world, in countries such as India and the Philippines. In the United States, the island of Moloka‘i in the Hawaiian archipelago contains the country's oldest asylum (see Father Damien and Kalawao County, Hawaii).
Western humanitarian and church organizations regularly send relief supplies, including handmade "leper bandages" to these colonies. Leper bandages are knitted or crocheted out of cotton, for better breathing than traditional gauze, and more durability—the bandages can be washed, sterilized, and reused. The bandages can be machine made, but the colony inhabitants appreciate handmade bandages.
In 2001, government-run leper colonies in Japan came under judicial scrutiny, leading to the determination that the Japanese government had mistreated the patients, and the District Court ordered Japan to pay compensation to former patients.  In 2002, a formal inquiry into these colonies was set up, and in March of 2005, the policy was strongly denounced. "Japan's policy of absolute quarantine... did not have any scientific grounds". Many children of those with Hansen's disease were executed by staff at colonies up to the 1950s. 
- Icon Health Publications. Leprosy: A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet References. San Diego: Icon Health Publications, 2004. ISBN 0597840067.
- Leprosy – World Health Organization
- Hansen's Disease (Leprosy) – Centers for Disease Control and Prevention
- INFOLEP Leprosy Information Services
- National Hansen's Disease Programs (NHDP) United States Department of Health and Human Services
- International Leprosy Association
- START – Leprosy Charity
- BBC News story: Slave trade key to leprosy spread