Ancient DNA isolated from the mass grave of a 15th century leper colony has given scientists a clue as to why the one-time scourge of humanity has all but vanished from western Europe. According to NPR’s Shots Health News blog, a paper published Thursday in Science magazine explained that human beings themselves changed to overcome the once-prevalent skin disease.

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Leprosy, also known as Hansen’s disease, afflicted as many as 1 in 30 citizens of Western Europe at its height in the 15th century. In Medieval woodcuts and drawings, lepers are represented almost as frequently as Christ and the Virgin Mary. But then, in the era after the Crusades, the disease mysteriously all but vanished from the continent.

Scientists wondered whether the bacteria that causes leprosy, Mycobacteriam leprae, had mutated into some less virulent form, or whether Europeans developed immunity. According to the study published in Science, it wasn’t the bacteria that mutated, it was the people.

Stewart Cole of the Ecole Polytechnique Fédérale de Lausanne, one of the study’s lead authors, told NPR that his team extracted DNA from the mass grave of a Medieval leper colony. DNA of any kind of is extremely difficult to extract from bones, but the team met with success when they were able to remove a small amount of tissue from a 600-year-old rotted tooth.

The material they found was “a mixture of human DNA, microorganisms and contaminating DNA from other bones and surrounding soil,” said Cole, but they were able to fully reconstruct a 600-year-old strain of leprosy and map its genome, only to find that it is essentially identical to living leprosy infecting people in the developing world today.

Cole told NPR, “If the explanation of the drop in leprosy cases isn’t in the pathogen, then it must be in the host, that is, in us.”

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The scientists believe that a certain gene that makes people highly resistant to leprosy spread through the population of Europe, gradually conveying a kind of mass immunity.

Today, leprosy infections are treated with antibiotics and can be handled quickly and effectively if caught early. Stigma associated with the disease often keeps people in the developing world from seeking help for the malady until it has run amok in the system, causing irreversible damage.

Cole told NPR that understanding a disease’s history can inform scientists’ and physicians’ approaches to treating and managing present and future diseases.

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“Having information about the specific genes and proteins in the disease can help to determine preventative and therapeutic strategies, as well as possible drug resistances,” he said.

[image of women dressed as Medieval lepers via Radu Razvan / Shutterstock.com]