The Bright Side of the Plague

What was the infectious agent of the Black Death that struck Europe in 1348 and succeeding decades? The classical answer is Yersinia pestis, today’s bubonic plague. But if the disease had been bubonic plague, then outbreaks in the human population should have been preceded by extensive deaths among local rodents. The fleas that transmitted the infection among rats would then have been forced to abandon the cold bodies of their former rodent hosts and would have settled on the people who fed and sheltered the rats. But no contemporary account mentions a lethal outbreak of mortal disease (or epizootic) among rodents during the fourteenth or fifteenth century. On these grounds, more than one historian has challenged the identification of the Black Death as bubonic plague. Still, the absence of evidence is not evidence of absence. Sometimes plague is transmitted from person to person by airborne droplets, and an initial epizootic among rodents could have passed unrecorded.

In his short, sweeping, and brilliant book1 the late historian David Herlihy brings new information from a most unlikely source to bear on the identification of the infectious agent of the Black Death. In the aftermath of the plague, the Church received many petitions to canonize persons who were said to have miraculously cured cases of the plague. In trials conducted and carefully documented by the Church, petitioners described the medical afflictions that were alleged to have been miraculously cured. The proceedings of many of these trials were collected in Acta Sanctorum, a seventy-volume hagiographic collection published in Antwerp starting in 1643.2 While some depositions mentioned the buboes, or characteristic lymph node swellings of bubonic plague, Herlihy points out that the “sign of the plague” frequently referred to in these trials is described as petechiae, or spots, characteristic of anthrax and some other diseases (but rare with bubonic plague).

Whether the symptoms of people who were miraculously cured should be taken as representative of the symptoms of the people who died of the plague—whatever it was—is not a question that Herlihy addresses. Miraculous survivals may have selectively favored people not in fact infected by fatal diseases.

Herlihy does not consider the historical evidence so far assembled to be decisive, and I agree. He suggests that rare mutant forms of plague are also consistent with the evidence. The example of HIV suggests to Herlihy that new infectious agents can appear apparently out of nowhere and can also disappear, leaving open the possibility that the agent of the Black Death may not be known to us now.

In October 1998, French scientists published new evidence that the infectious agent of the Black Death was indeed today’s bubonic plague, at least from the sixteenth century onward.3 Urban construction projects in southern France had recently uncovered two mass graves known from historical evidence to contain victims of quarantine hospitals for what was then called “plague.” To avoid possible contamination by any other source, the French team pulled teeth that had never emerged from the…

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