 Thus environmental stressors are considered when hypothesizing the deadliness of the Black Plague, such as crop failures due to changes in weather, the subsequent famine, and an influx of host rats into Europe from China.The Black Death was so devastating that a comparable plague in terms of virulence had not been seen since the Justinian plague, before the Medieval warm period.This gap in plague activity during the Medieval Warm Period contributes to the hypothesis that climate conditions would have affected Europe's susceptibility to disease when the climate began to cool during the arrival of the Little Ice Age in the 13th century.