Scientists have reconstructed ancient strains of leprosy to explain the disease’s sudden decrease at the beginning of the 16th century. It turns out that human evolution played an important role in leprosy’s medieval disappearing act.

Researchers from Switzerland’s Ecole Polytechnique Fédérale de Lausanne and Germany’s Tubingen University exhumed bodies from medieval graves and collected strains of Mycobacterium leprae, the pathogen that causes leprosy. According to their report, leprosy was common during the Middle Ages, with approximately one in 30 people affected by the disease. Leprosy still occurs today, with approximately 200,000 people affected by the disease each year.

Lepers were shunned or sent to isolated colonies for treatment and to contain its spread. While leprosy may have been more common during the Middle Ages, the disease’s occurrence began to mysteriously decrease in Europe. The team of researchers was able to reconstruct five strains of the bacteria by carefully separating the disease’s DNA from human DNA. The precise method developed by the researchers allowed for the re-creation of ancient mycobacterium strains.

According to co-author Pushpendra Singh, from EPFL, “We were able to reconstruct the genome without using any contemporary strains as a basis.” This method of reconstruction allowed for a comparison between ancient strains and contemporary strains of the disease. The researchers discovered the two strains were identical and therefore ruled out the possibility of the mycobacterium mutating into a less potent form.

Ruling out a change in the pathogen, co-leader Stewart Cole, from EPFL, explains that the sudden drop in leprosy cases must be due to changes within humans. Cole says, “If the explanation of the drop in leprosy cases isn't in the pathogen, then it must be in the host -- that is, us -- so that's where we need to look.”

The researchers believe leprosy declined due to natural selection as humans developed a resistance to it. Cole says the isolation of lepers, as well as pressure on them not to procreate, could have limited the disease from spreading. He also notes, “Other studies have identified genetic causes that made most Europeans more resistant than the rest of the world population, which also lends credence to this hypothesis.”

In addition to solving this ancient mystery, the tools developed by the researchers can be used for future examinations of the disease. Future research could use genetic sequencing to determine leprosy’s origins or discover new potential pathogens. The study was published in the journal Science.