The Black Death of 1347 was believed to be the third great outbreak of bubonic plague; a plague that is traditionally spread by rats and fleas. The previous instances were the Plague of Athens in 430 BC and the plague at the time of Justinian which arrived into Constantinople in AD 542. The Plague of Athens was described by Thucydides, while the Justinian plague was described by Procopius, among others. [...]

The plague is supposed to have originated in Central Asia, or somewhere in Africa, where plague is endemic in some rodent populations. It is assumed that some environmental stimulus caused infected rodents to leave their normal habitats and infect rat populations, and ultimately human populations, in areas where there was no natural immunity. The mechanism of transfer is believed to have been infected fleas leaving the bodies of dead rats and moving to human hosts who were in turn infected by the feeding fleas. It is believed that trade routes brought the disease to the Black Sea region and from there to the central Mediterranean by late 1347. It was then introduced into Europe through northern Italy and southern France. It immediately started killing people in large numbers spreading overland at about 1.5 km per day. Between January and the summer to autumn of 1348 it had spread as far as the British Isles, and by 1350 to Scandinavia and eventually even Iceland. The spread seems to have curled up through France, across Belgium into Germany and on into central southern Europe. This first wave burned itself out by 1351, though there was a second wave in 1361.

It is generally believed that the plague hit an already weakened population in Europe. [...]

At its most basic, the problem is with those rats and fleas. For the conventional wisdom to work there have to be hosts of infected rats and they have to be moving at alarming speed - you would almost have to imagine infected rats scuttling every onward (mostly northward) delivering, as they died, loads of infected fleas. The snags with this scenario are legion. For example, there are no descriptions of dead rats lying everywhere (this is explained by suggesting that either the rats were indoors, or people were so used to dead rats that they were not worth mentioning; though if they were indoors how did they travel so fast?) It did not seem to matter whether you were a rural shepherd or cleric or a town dweller, both were infected. Yet strangely with this very infectious disease some cities across Europe were spared. Moreover, these rats must have been happy to move to cool northern areas even though bubonic plague is a disease that requires relatively warm temperatures. Then, when there are water barriers, these rats board ships to keep the momentum going. (Baillie)