AFTER THE JUSTINIANIC Plague (c. 541–c. 750), which has been called the First Plague Pandemic, the Black Death or Second Plague Pandemic was likely the first semi-global phenomenon that fully merits the name—affecting “all people” (pan + demos). Total (absolute) mortality would be higher from several nineteenth-century cholera outbreaks, the 1918–19 influenza pandemic, or the current HIV/AIDS pandemic. But when expressed as a percentage of the population, the mortality caused by the Black Death is the highest of any large-scale catastrophe known to humankind, save for the impact of smallpox and measles on indigenous peoples in first-contact events of the early modern period. The Black Death killed an estimated 40% to 60% of all people in Europe, the Middle East, and North Africa when it first struck there in the mid-fourteenth century. Its demographic effects are well known (particularly with respect to Western Europe), and there is a considerable body of historical scholarship on population losses and the economic and political changes that ensued. Such questions about its aftermath are important, of course; but so, too, are questions of why and how the pandemic happened in the first place and how it was sustained. For these questions, we currently have no definitive answers. Even its full geographic extent is still unknown: we are only now beginning to engage with scientific and documentary evidence for plague in East Asia, and we can only suspect (as we will see) whether the disease might have also reached other regions of Afroeurasia as well.

A catastrophe of this magnitude demands explanation. The present collection of essays starts from the premise that studies of the Black Death must embrace a new reality: the fact that the field of microbiology has, in the past two decades, leapt past the barriers of the late nineteenth-century biological laboratory and created new ways to explore the history of pathogenic organisms. Microbiology has effected a transformation in our understanding of the disease's history. It has reconstructed the phylogenetic (evolutionary) history of the plague organism, Yersinia pestis, from modern molecular samples, and it has developed techniques to retrieve and reconstruct genetic material of the pathogen from historical remains. The question “What was the pathogen?”