"Straining the History of Infectious Diseases: Europe's Two Black Deaths"

This is the abstract for the Garrison Lecture I will be giving on 5 June 2026 at the annual meeting of the American Association for the History of Medicine. The Garrison Lecture is the invited plenary lecture of the AAHM, given every year by "a scholar distinguished for contributions to medical history or other fields of science and learning." The lecture "presents original and previously unpublished research." The present lecture, entitled "Straining the History of Infectious Diseases: Europe's Two Black Deaths," examines how new information about genetically distinguishable strains of Yersinia pestis (the bacterium that causes plague) can be used as the foundation for a new understanding of medieval Europe's experience of plague during what we have come to know as the Black Death. The lecture will argue that Europe experienced two separate waves of plague: one in the north caused by a lineage of plague that had already entered Europe in the later 13th century; and another in the south, caused by a more virulent lineage of plague recently arisen in the former lands of the Mongol Ilkhanate. Both strains peaked at more or less the same time between 1346 and 1350 because recent volcanic activity in 1345 created favorable circumstances for plague's re-emergence. Together, these "two Black Deaths" caused greater mortality than any other pandemic in human history and instigated a likewise unprecedented series of persecutions.