How did Black Death spread?

The Black Death had been endemic to Asia, but came to
Europe by means of ships landing in Italy which originated in Asia. The bacterium
responsible for the Black Death, yersenia pestis only survived on a
certain type of flea that only survived on Asian black rats. European brown rats were
not a suitable host.The rats were transmitted to Europe by means of ship, and thereby
carried the disease to Europe. A ship which landed in Messina in October, 1347 is widely
suspected as the first to transmit the rats to Europe. Town officials would not allow
the ship to offload, as the sailors were either dead or dying; yet in the time the ship
was docked, the Asian rats left the ship and made their way inland. Fleas from the rats
often infested the bedding and clothing of people of the European Middle Ages and bit
their human hosts, thereby inflicting them with the plague. The clothes of the deceased
were often passed on to others who were also bitten by the fleas. Numerous remedies,
including inhaling vapors from urinals, were attempted to stop its spread, all to no
avail.

The plague, known as astra mors
(dreadful death) but later "black Death had two phases:
bubonic, which could only be transmitted by flea bites, and
pneumonic which could be transmitted from human to human. The
plague seldom reached the pneumonic phase, as victims normally died during the bubonic
stage. An often misunderstood fact is that the Plague did not descend on Europe as a
blanket but rather as a cascading wave. Much of the panic caused by the plague was in
towns and villages where it had not yet struck; but whose inhabitants saw it coming and
could do nothing other than wait. Plague normally passed through a given area within two
weeks, yet it left unprecedented death in its wake.

Plague
finally passed when the European Brown rats drove off their black rivals; but before it
ended, fully one fourth the population of Europe had died as a
result.