There are no facts, only interpretations of facts.

Introduction

Lester Little’s brilliant and prescient 2011 review article ‘Plague Historians in Lab Coats’ ends with the first draft genome of the ancient pathogen Yersinia pestis, the causative agent of plague, being reconstructed from ancient DNA (aDNA). The pathogen was found in the teeth of human remains discovered in a mid-fourteenth-century burial site in East Smithfield, London. In the article, Little addressed the emerging gap between historians, who had occupied the retrospective plague space for decades, and geneticists all too eager to claim this space as their own. With modern techniques paving the way for ancient pathogen detection, geneticists felt well placed to bring the years-long debates about the causative agents, origins, timing of the emergence and disappearances of past plagues to a close. Seen in hindsight, Little’s chronicling of the discovery of the aetiological agents of historical plague outbreaks is rather like a brisk walk – nothing like the frenetic pace of new discoveries today. His paper echoes back to the early days of germ theory and microbial identity, which were synonymous for disease at the time. In 1873, the Norwegian physician Gerhard Armauer Hansen discovered the leprosy bacillus. Subsequent discoveries, including those by Robert Koch and Louis Pasteur, not only informed thinking about modern disease but also were often applied retrospectively to the past with no evidence to either support or refute such applications. It was not until more recent palaeopathology and aDNA work enabled definitive proof of the presence (but not the absence) of some particular pathogens that somewhat less tenuous retrospective histories could be offered.

Although the advent of new scientific methodologies has enabled positive identifica-tion of pathogens responsible for infectious disease that have long been visible in bone (e.g., leprosy and tuberculosis) or presumed present (e.g., plague) and have been instru-mental in rendering the once-invisible (e.g., salmonella and Hepatitis B virus (HBV)) perceptible, they nevertheless still require appropriate context to be useful and meaningful in the broader scholarship. In short, while these scientific techniques revealed new facts, they also generated additional challenges with interpretation. One must be cautious with the presumed power of a DNA sequence.