The Black Death's Genetic Legacy: How Plague Shaped Our DNA

A Selective Event, Not Just a Demographic One

Between 1347 and 1353, the bacterium Yersinia pestis swept across Europe in the pandemic known as the Black Death. The mortality was staggering: an estimated 30 to 60 percent of Europe's population died within a span of roughly six years. Entire villages were depopulated. Cities lost half their inhabitants. The social, economic, and political consequences lasted centuries.

From a genetic perspective, the Black Death was long understood primarily as a population bottleneck — a massive reduction in population size that would have reduced genetic diversity and shifted allele frequencies through random chance. But a landmark 2022 study published in Nature by Klunk and colleagues revealed something more specific: the Black Death was not just a bottleneck. It was a selective event. The plague did not kill randomly. People carrying certain genetic variants were more likely to survive, and those variants became more common in the post-plague population.

The implications reach all the way to the present day.

The Evidence: Ancient DNA Before and After

The 2022 study took a direct approach. The researchers extracted and sequenced DNA from 206 individuals buried in London and Denmark — some who died before the Black Death, some who died during it, and some who lived in the decades after. By comparing allele frequencies across these three time periods, they could identify genetic variants that changed frequency during the plague years.

The most dramatic finding centered on a gene called ERAP2. This gene encodes a protein involved in the immune system's ability to present pathogen-derived proteins to T cells — part of the mechanism by which the body recognizes and fights infection. A specific variant of ERAP2 (rs2549794) was significantly more common in post-plague populations than in pre-plague populations. Individuals who carried two copies of the protective variant were estimated to have been approximately 40% more likely to survive the Black Death than individuals who carried two copies of the alternative variant.

A 40% survival advantage is an enormous selective pressure — one of the strongest documented in human evolutionary history. For comparison, the selective advantage of the lactose tolerance mutation in pastoral societies is estimated at roughly 1-10% per generation. The Black Death exerted more selective pressure in a few years than most environmental factors exert across centuries.

How Plague Killed — and How Genetics Helped

Yersinia pestis kills through overwhelming bacterial infection. The bacterium enters the body through flea bites (bubonic plague), inhalation (pneumonic plague), or the bloodstream (septicemic plague). It evades the immune system using a set of virulence factors that suppress the body's inflammatory response — essentially shutting down the immune defenses that would normally contain the infection.

The genetic variants that conferred survival advantage during the Black Death appear to have worked by enhancing the immune system's initial response to the bacterium. The ERAP2 variant, specifically, improved the ability of immune cells to process and present Yersinia pestis antigens — allowing the adaptive immune system to mount a faster and more effective defense before the bacterium could establish overwhelming infection.

Other immune-related genes also showed significant frequency shifts across the plague period, though none as dramatic as ERAP2. The overall pattern suggests that the Black Death selected for a stronger, faster-responding immune system — rewarding genetic variants that kept the body's defenses active in the face of a pathogen that evolved specifically to suppress them.

The Autoimmune Trade-Off

Here is where the genetic legacy of the Black Death becomes directly relevant to modern health. The same ERAP2 variant that protected against plague in the fourteenth century is, in modern populations, associated with increased susceptibility to autoimmune diseases — particularly Crohn's disease and other inflammatory bowel conditions.

This is not a coincidence. It is a direct trade-off. The immune system has two failure modes: it can respond too weakly (allowing infections to overwhelm the body) or too strongly (attacking the body's own tissues, producing autoimmune disease). The Black Death selected for a more aggressive immune response. That aggressive response was lifesaving during a Yersinia pestis pandemic. In the absence of plague, it increases the risk of the immune system overreacting to benign stimuli — producing chronic inflammation and autoimmune pathology.

This trade-off is a textbook example of balancing selection — the evolutionary process by which alleles that are advantageous in one context are disadvantageous in another. The plague created a selective environment in which the autoimmune risk was worth the survival benefit. Once the plague receded, the cost remained while the benefit diminished. The elevated frequency of autoimmune-associated alleles in modern European populations is, in part, a legacy of a selective event that occurred nearly seven hundred years ago.

Plague and Population Genetics

Beyond the specific immune gene findings, the Black Death left broader marks on European population genetics.

The mortality was not geographically uniform. Some regions lost 80% or more of their population; others lost 20% or less. This differential mortality created regional genetic bottleneck effects — small surviving populations that disproportionately shaped the gene pool of subsequent generations. In regions with the highest mortality, genetic diversity was reduced and founder effects from the surviving population are potentially detectable.

The plague also had secondary genetic effects through its social and economic consequences. The massive labor shortage that followed the Black Death improved the bargaining power of surviving peasants, increased social mobility, and disrupted traditional marriage patterns. These social changes may have altered gene flow patterns — breaking down the geographic and social barriers that had previously limited who married whom in medieval European communities.

Reading the Plague in Modern Genomes

The genetic legacy of the Black Death is not a historical curiosity. It is a living presence in the genomes of modern Europeans. The immune gene variants selected by plague are carried by millions of people today. The elevated rates of Crohn's disease and other autoimmune conditions in European-descended populations are, in part, the price paid for an immune advantage that saved millions of lives in the fourteenth century.

This finding also carries a broader lesson for genetic genealogy and ancestry research. Your genome is not just a record of who your ancestors were — it is a record of what they survived. The alleles you carry were shaped by the selective pressures your ancestors faced: plague, famine, climate, and disease. Every survival advantage came with a trade-off, and those trade-offs are still playing out in the health profiles of their descendants.

The Black Death killed perhaps 75 to 200 million people across Eurasia. The survivors were not a random sample. They were selected — by a bacterium that did not care about rank, wealth, or culture, but that was, in a measurable and documented way, influenced by the alleles its victims carried. That selection is the plague's most lasting legacy.