Scientists have discovered a gene variant which helped protect some humans from developing the Black Death is now causing their descendants auto-immune issues.
It is 700 years since the plague wiped out millions, and investigations studying mass graves have revealed mutations that helped people survive the plague, which hit Europe in the mid-1300s.
Analysis of DNA from skeletons have uncovered the variants which gave some people added protection.
However, the same mutations are linked to auto-immune diseases, such as the inflammatory bowel disease Crohn's, which are affecting people today.
DNA was taken from the teeth of 206 ancient skeletons from mass graves in London's East Smithfield plague pits and others from Denmark.
“This pandemic devastated Afro-Eurasia, killing up to 30 per cent to 50 per cent of the population,” the report says.
“To identify loci that may have been under selection during the Black Death, we characterised genetic variation around immune-related genes from 206 ancient DNA extracts, stemming from two different European populations before, during and after the Black Death.”
The data, which has been published in the Nature journal, revealed that a gene called ERAP2 gave people a 40 per cent better chance of surviving the plague.
“Another of our top candidate loci is associated with an increased risk of rheumatoid arthritis and systemic lupus, such that retaining the putatively advantageous allele during the Black Death confers increased risk for autoimmune disease in present-day populations,” the report says.
Professor Luis Barreiro, from the University of Chicago, told the BBC, that the gene's 40 per cent plague survival rate was the “strongest selective fitness effect ever estimated in humans”.
“That's huge, it's a huge effect, it's a surprise to find something like that in the human genome,” Professor Barreiro said.
Earlier this year it was discovered that the origins of the Black Death, the plague that caused a pandemic lasting hundreds of years, could be pinpointed to a lake region in modern-day Kyrgyzstan in 1338.
Less than a decade later, the plague first entered the Mediterranean on trade ships transporting goods from the territories of the Golden Horde in the Black Sea.
The disease then spread across Europe, the Middle East and northern Africa, killing up to 60 per cent of the population.
This first wave became a 500-year pandemic, the so-called Second Plague Pandemic, which lasted until the early 19th century.
