As the Black Death swept through Europe in the mid-14th century, many Christians, observing that their Jewish neighbors tended to die at less alarming rates, decided that the Jews were to blame. This conclusion resulted in some of the Middle Ages’ severest outbreaks of anti-Semitic violence. Over the last 200 years, historians have variously pinned the difference in mortality to better hygiene (perhaps related to handwashing and other halakhic observances), more fastidious care for the sick, kashrut, and Passover cleaning. Declaring these explanations scientifically untenable, Kathryn Glatter and Paul Finkelman suggest a different one:
We believe that the answer lies in a recessive genetic mutation—familial Mediterranean fever (FMF), which is found mostly in people of Middle Eastern ancestry. In 14th-century Europe, the most prominent and visible group of such people would have been Jews. FMF causes recurrent fevers and painful inflammation of the abdomen, lungs, and joints. However, as a 2020 study at the National Human Genome Research Institute showed, it also makes its carriers resistant to the bubonic plague.
A recent analysis of 400 blood samples in Tel Aviv found FMF in 39 percent of Jews of Iraqi origin, 22 percent of Jews from North Africa, and 21 percent of Ashkenazi Jews. In another Israeli study, 14 percent of Jews of Turkish origin and 20 percent of Jews from Bukhara, Georgia, and Bulgaria carried the mutation. Altogether, it seems that 20 to 40 percent of Israeli Jews might carry the FMF mutation.
Since FMF is a recessive gene, its carriage rate has likely declined since the 14th century, meaning that many more Jews would have carried the FMF mutation at the time of the plague. If so, this would explain the contemporary impression that Jews survived the plague in disproportionate numbers compared with their Christian neighbors. The fact that thousands of them were then murdered, in part because of their congenital immunity, is one of the tragic ironies of history.