The Second Plague Pandemic of the mid-14th century, also known as the Black Death, killed 30% to 60% of the European population and profoundly changed the course of European history. A new study led by researchers from Penn State and the University of Adelaide suggests that this plague, potentially through resulting changes in diet and hygiene, may also be associated with a shift in the composition of the human oral microbiome toward one that contributes to chronic diseases in modern-day humans.