In the 14th century, the Black Death swept through North Africa, the Middle East, and Europe, wreaking havoc on local populations. It killed indiscriminately and without mercy, striking down between 30 and 60 percent of the populations it infected.

But did you know the plague continues to impact the human population today? Keep reading to find out more about its “biological legacy.”

Horrifying History

Also known as bubonic plague, it comes in two primary varieties: septicemic and pneumonic. The first infects the blood with bacteria and the second attacks the lungs. Both prove deadly unless treated early.

Of course, in a world without antibiotics or even a clear understanding of how diseases spread, the kill rate proved devastating. Europe saw more than 50 million individuals perish from it. In some cases, it wiped out whole families or even villages.

We now know the disease was caused by the bacteria Yersinia pestis, usually found in small mammals and fleas. And while you might think the Black Death sits firmly in the past, you might be surprised to find out how it continues to affect humans in modern times.

The Plague’s Biological Legacy

Researchers at the University of Chicago, the Pasteur Institute in Paris, and the McMaster University in Ontario, Canada, recently announced a stunning find — one that’s rewriting how we view the history of the bubonic plague and the human genome.

According to these scientists, the bubonic plague had a massive impact on the human immune system, and is still being felt today.

These researchers painstakingly sampled the bones of more than 200 people from Denmark and the British Isles. The individuals involved in the sampling lived within a century of the plague. What did the scientists ultimately find? Four genes that dictated how likely a given individual was to die of the Black Plague.

Bubonic Plague and Autoimmune Disorders

Surprisingly, the scientists discovered that the very genes that helped medieval people survive the plague contribute to a host of other conditions today. These ailments include lupus, Crohn’s disease, and rheumatoid arthritis. All of these diseases share something in common: they are autoimmune conditions caused by the immune system attacking itself.

Hendrik Poinar, an anthropology professor at McMaster, puts it this way: “A hyperactive immune system may have been great in the past but in the environment today it might not be as helpful.” That’s an understatement.

Autoimmune Disorders Today

Today, there are more than 100 autoimmune diseases impacting more than 50 million Americans daily. Put another way, one in five Americans have some form of autoimmune problem. If researchers are correct, many (if not most) are descendants of plague survivors.

The scientists’ colleagues have lauded this research as a significant breakthrough. At the same time, it’s also hard to deny the strange irony associated with this discovery. After all, it’s mindboggling to realize the same gene that helped one generation dodge the Yersinia pestis bullet now makes their descendants chronically ill.

By Engrid Barnett, contributor for Ripleys.com

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