for National Geographic News
It is an infectious disease that dates back at least to biblical times, yet leprosy has puzzled scientists since the identification, in 1873, of the bacterium that causes it.
Known for its disfiguring skin lesions and potentially debilitating nerve damage, leprosy, or Hansen's disease, is a very difficult disease to transmit. It also has a long incubation period, making it hard for a doctor to determine where a leprosy patient contracted the disease.
But now a team of French scientists has discovered how the disease evolved and how it was spread across the continents by human migrations.
The scientists found that leprosy infections were caused by a single bacterial clone, which spreadbut barely mutatedfor centuries. Such behavior is highly unusual.
Researchers also found that leprosy probably originated in East Africa and not India, as previously thought. The disease was brought eastward and westward by colonialism and the slave trade, the scientists believe.
"The bacterium has a highly stable genome and appears to have been spread between people by contact or the aerosol [airborne particles] route and dispersed around the world by human migrations," said Stewart Cole, a geneticist at the Pasteur Institute in Paris, France.
Cole is a co-author of the study, which is reported this week in the academic journal Science.
For centuries leprosy was incurable and severely disfiguring. Lepers were usually shunned and sequestered in leper colonies.
The disease produces lesions on the skin. The most severe form of leprosy produces large disfiguring nodules, or lumps. The disease can also cause nerve damage in the extremities, sensory loss in the skin, and muscle weakness.
Although leprosy is easily curable by antibiotic therapy today, the disease is still common in many countries in the world, especially in tropical climates. About a hundred cases per year are diagnosed in the United States.
An unusual aspect of leprosy infection is the disease's slow incubation period, which lasts several years.
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