Survival of the fittest determined who lived through the plague. But those very genes may make us susceptible to other diseases.

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THE FOURTEENTH CENTURY was a rough time to be alive. Not so much because of the time itself but because of what was happening: the Black Death. Arguably the deadliest pandemic in human history, the Black Death descended from an ancient bubonic plague that ravaged Rome in the mid-6th century and lingered throughout the centuries by mutating into a form more virulent to humans.


The plague is caused by the bacterium Yersinia pestis, which is transmitted by infected fleas to rats. The second pandemic of the plague exacted a heavy toll, killing an estimated 30 to 50 percent of the world’s population. It also left an indelible mark on its survivors and their descendants, it turns out — an evolutionary struggle encoded deep within our DNA.

In a study published Wednesday in the journal Nature, a group of researchers has found that the Black Death may have shaped the human immune system, specifically by favoring (inadvertently, of course) survivors with robust genes that protected against the vicious germ. Over 600 years later, these Black Death-proof immune genes still circulate within us and may have repercussions today in the 21st century.

“Understanding the dynamics that have shaped the human immune system is key to understanding how past pandemics, like the plague, contribute to our susceptibility to disease in modern times,” Hendrik Poinar, an evolutionary geneticist at McMaster University’s Ancient DNA Centre and co-author of the paper, said in a press release.

Rows of unearthed graves exposing skeletons inside
Researchers extracted DNA from the remains of people buried in the East Smithfield plague pits, which were used for mass burials in 1348 and 1349.Museum of London Archaeology (MOLA)

HOW THEY DID IT — The researchers — a collaboration of scientists from McMaster, the University of Chicago, the Pasteur Institute in France, and other organizations — took bone samples from three burial sites in London (including the infamous East Smithfield plague pit) and five across Denmark.