The plague pandemic influenced the evolution of human immunity by determining how we respond to infections today, scientists from McMaster University found.  Their findings are published in the journal Nature. The bubonic plague pandemic gripped Europe, Asia and Africa almost 700 years ago. It killed more than half the population in the most populous parts of the world at the time. By analyzing DNA extracted from the teeth of people who died before and during the Black Death pandemic, researchers were able to find genetic differences that affected survival from the virus. The researchers estimated that people with a protective variant of the ERAP2 gene were 40-50 percent more likely to survive than those who didn't have it. It turned out that the same genes that once provided protection against the plague are now linked to increased susceptibility to autoimmune diseases such as Crohn's disease and rheumatoid arthritis. When a pandemic of this kind occurs, killing 30 to 50 percent of the population, there is necessarily a selection of protective genetic variants in people, meaning that people who are susceptible to the circulating pathogen die. Even a small advantage means the difference between surviving or passing. Of course, those survivors who have reached childbearing age will pass on their genes, which will take hold in the population, explains Luis Barreiro, a geneticist at the University of Chicago in Illinois.