For most of the 20th century, historians accepted bubonic plague, Yersinia pestis, as the cause of the 14th-century Black Death, although a wealth of primary sources seems to conflict with that diagnosis. Scientists focused on Y. pestis have been less certain than historians, arguing over their findings and how these might apply to the 14th century. Historians who believe the diagnosis was premature point to the speed with which the disease travelled, and killed; its symptoms, and its extraordinary morbidity and mortality rates; the shortage of evidence for rats or fleas; and the seasons and climates in which the disease thrived; and they question whether every inconvenient bit of medieval evidence can be covered by later hypothetical mutations. Yet, guided by textbooks, and by popular media that distort the work of scientists, teachers often omit medieval evidence that doesn't fit Yersinia. Uncertainty over the cause of the Black Death offers historians a chance to show how historical knowledge is generated, by realistic use of primary sources in dialogue with other scholarly disciplines.