New Definition of the Black Death: Genetic Findings and Historical Interpretations

The field of infectious disease history has been transformed in the past decade in large part because of fortuitous developments in several adjacent fields, most importantly genetics. The medieval period (ca. 500 to ca. 1500) has proved particularly important for these developments, not simply because it is now the earliest period from which whole genomes of several bacterial and viral pathogens have been retrieved, but also because the narratives that can be constructed about disease emergence and dissemination are most robust for this period thanks to the amount of surviving archival evidence. This essay introduces the transformative work in molecular biology that has allowed reconstruction of the evolutionary histories of pathogens afflicting humankind. Plague, the disease caused by the bacterium Yersinia pestis, has been at the forefront of this new work. An extraordinarily lethal disease, plague gives a prime example of how the evolutionary narratives of genetics yield information valuable to historians, precisely because they allow us to see phenomena at a hitherto invisible microscopic level. This essay proposes a new definition of the Black Death, or more properly, the Second Plague Pandemic, which is based on a new, evolutionary understanding of the pandemic’s pathogen. However, scientific approaches are not inherently more productive of unassailable truths than are traditional humanistic or archaeological methods for the study of disease history. The complementarity of material and cultural sources is especially fruitful for work employing the perspectives of global history. Although most historiography on the late medieval pandemic has focused on Europe, whose crisis with plague did not begin until 1347, I argue that such a limited geographic definition occludes not only as much as a century and a half of plague activity, but also occludes connected events in Asia and Africa. Given the urgency of understanding pandemics in a world of rapid change, the history of plague offers us the best available model for thinking about pandemics.