War and famine were not the only things that humans had to fear in the Middle or Dark Ages. Disease was little understood and the life expectancy of the average human was relatively lower when compared with later ages.
The Black Death was thought to have originated on the plains of Central Asia and was brought to the mainland of England by the Silk Road which was the route that the Asian traders used to bring goods to trade to Crimea that were then loaded and brought to different parts of the world.
The Black Death or the Plague resulted in the death of more than 100 million people worldwide, and for the reduction of approximately 30-60% of the population of Europe alone.
The disease was carried by the fleas that attached themselves to the rodents in many parts of Asia, and was first reported in Europe in around 1347 when Genoese traders who had fled the siege of Mongol leader Jani Berg at Kaffa landed in Sicily and Southern Europe.
Berg in attempts to cause capitulation of forces holed up in castles and barricaded fortresses launched the bodies of diseased ridden soldiers of his force who were suffering from the affliction into the castle and fortresses of his enemies not knowing that the disease would spread so rapidly and cause the deaths of so many.
From the earliest reports in Italy, the disease then spread across Europe hitting the population of France, Spain, Portugal and then into England. It continued to Germany and the Scandanavian countries and finally reached the shores of Russia in about 1351.
While the entire death toll will never be completely accurate as there was no way to know exactly how many people populated the lands that suffered outbreaks, the general consensus is that the Black Death killed approximately 75-200 million people worldwide before the plague died out.