It was said that the cause of the Pestilence or The Great Mortality — 14th-century names for the contagion — was a particularly sinister alignment of the planets, or a foul wind created by recent earthquakes. Other theories existed. "Looks," according to one medieval physician, "could kill."

But the source of the pestilence was something much more common and much more insidious — rats and fleas infected with plague.

For centuries the bacillus Yersinia pestis, the bacteria associated with plague, lived comfortably within the confines of the blood streams of the small medieval wild black rat and the stomachs of adult rat fleas, and for centuries human populations were left untouched. The habitats of wild rats and humans rarely crossed paths, and rat fleas seldom found the blood of humans an enjoyable meal. People were accidental victims when no other warm-blooded small mammal was available.

Uncertainty Still Exists
Uncertainty still exists as to why the disease spread with such virulence at this point in history, but severe ecological changes in Central Asia in the mid-14th century could have contributed. Medieval chroniclers write about famines, floods and earthquakes. Such disturbances may have driven infected rats scurrying into human settlements, allowing the disease to become endemic in the common urban or sewer rat. The plague exists even today and just as in the 14th century, the disease spreads primarily through the bite of a flea.

Rats are merely the vehicle for transporting fleas from one area to another; rats, like humans, eventually contract the disease and die, forcing diseased fleas to seek another warm-blooded mammal to survive. In the 14th century, hidden among the silks and grains in the hull of a ship, hungry fleas waited for an unsuspecting seaman to unload the precious goods. To make room for their next bloody meal, fleas, engorged with Yersinia pestis, regurgitated the swarming bacillus within their stomachs into the bloodstreams of their victims.

Rats and fleas have been the barely noticed companions of sailors ever since the time humans erected sails on wooden boats. In the mid-14th century the plague was ready to be shipped to the world. The only question was "Where would it strike next?"