While some scholars have questioned whether the pandemic known as the Black Death was plague at all, Benedictow gives little credence to this theory. He shows that the medieval pandemic is closely related to modern outbreaks of plague that are caused by the bacteria yersinia pestis. While some argue that the Black Death was devastating because of multiple carrier species—that is, both bubonic and pneumonic plague—Benedictow’s epidemiological evidence and written descriptions from the period suggest that pneumonic plague’s impact during the medieval pandemic was negligible.

For example, the fact that evidence shows the plague’s pace of spread consistently slowed during cold months and in mountainous regions where temperatures were lower confirms that this was bubonic plague. Pneumonic plague spreads regardless of the season. Bubonic plague flourishes in warm and damp conditions, which not only explains why the evidence shows it spreading more rapidly during the spring and summer months but also explains why ship transport was the major mode by which the Black Death moved. Conditions at sea are humid and, thus, foster the transmission of the bubonic plague. For example, Italian merchants fleeing the city of Kaffa on the Caspian Sea brought the plague westward on their ships.