"Agnolo di Tura was a sometime shoemaker and tax collector with a yen for keeping a journal. He was also his family’s sole survivor when the Black Death tore through Siena, Italy, in 1348. He buried his wife and five children with his own hands, he wrote in his journal. He was somehow spared.
Many deadly diseases have swept through the human population over the past 10,000 years, claiming some, leaving others behind. These tragic events left their mark on our DNA. When biologists compare modern genomes to DNA extracted from ancient bones, they can see how genetic variants that enabled people to fight off pathogens have increased in frequency over millennia.
We are the descendants of the survivors, and our genomes show it. But it turns out this genetic history can be a double-edged sword.
In work published in the journal Cell Genomics, researchers found that many of the protective variants that have increased in frequency also raise the risk of autoimmune diseases, in which the immune system turns on the body. It’s an intriguing thought: Over the course of human evolution, what saves you might come back to haunt your descendants."