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Genes that gave our ancestors a life-saving advantage during the Black Death are responsible for higher rates of autoimmune diseases such as Crohn’s and rheumatoid arthritis today, scientists say.

Their study has identified specific gene variants that gave certain people significantly better odds of surviving the bubonic plague (Yersinia pestis) pandemic that killed upwards of half the population in large parts of Europe, Asia and Africa during the 1340s and 1350s.

The researchers said that, in revealing that these variants became far more prevalent as a result of the pandemic, the study was the first hard evidence that the Black Death played an important role in the evolution of the human immune system through natural selection.