The Black Death is the name most commonly given to the pandemic of bubonic plague that ravaged the mediaeval world in the late 1340s. From Central Asia the plague swept through Europe, leaving millions of dead in its wake. Between a quarter and a third of Europe's population died. In England, the population fell from nearly six million to just over three million. The Black Death was the greatest demographic disaster in European history. Sean Martin looks at the origins of the disease and traces its terrible march through Europe.