Comment: There is another take on what caused "Black Death", which is backed up by substantial evidence, considering serious problems with original explanations of the disease spreading rate.

From New Light on the Black Death: The Cosmic Connection

[T]he Black Death, one of the most deadly pandemics in human history, said to have killed possibly two thirds of the entire population of Europe, not to mention millions all over the planet, probably wasn't Bubonic Plague but was rather Death By Comet(s)!

Oh yeah! That's far out, isn't it?

Maybe not. Baillie has the scientific evidence to support his theory and his evidence actually supports - and is supported by - what the people of the time were saying: earthquakes, comets, rains of death and fire, corrupted atmosphere, and death on a scale that is almost unimaginable.[...]

Baillie sums up the problem as follows:

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. [...]

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)