The first time plague affected Western Europe was in the early Middle Ages: rom 541 to 767, there were no fewer than 15 outbreaks in southern parts of the continent. Plague then disappeared from Europe for some seven centuries but came back with a vengeance in 1347, this time by way of the Mediterranean, and ravaged the entire continent for five years, resulting in a serious demographic depression. From then on until 1722 (and 1771 in Moscow), the disease remained endemic to Europe, periodically undermining its economy. These epidemics were major determinants of medieval history, but their study has not been completed to this day. It was not until the 1970s that archeo-zoologists finally discovered that the black rat had indeed been present in Europe since Roman times. Further extensive research revealed that the rat population had gradually grown from a fairly restricted one in the early Middle Ages to a significant one in the 11th and 13th centuries. The rodents spread along the major highways explaining the very different geographical impact of the various plague epidemics of the early and late medieval periods. However, the mystery of the exact mechanisms by which plague spread has still not been entirely elucidated, since the Asian rat flea, Xenopsylla cheopis, whose role as vector was demonstrated by P. L. Simond, could not have survived in the temperate European climate. Thus, the question of the European vector is still left hanging: was it a human or a rat flea? Was the rat a propagator or simply an initiator? This article considers these unresolved questions by re-examining P. L. Simond's very precise observations.