Now, researchers say newly analyzed evidence suggests an additional, unexpected reason for the plague’s medieval resurgence. “What were the drivers of the onset and transmission of the Black Death, and how unusual were they? Taken together, the written sources and tree rings imply nearby volcanic activity around 1345. Once in Europe, the fleas transferred to rodents and the Black Death began its infamous spread. A warning for todayThe effects of the Black Death in Europe famously weren’t uniform.