The Black Death is generally understood to have been caused by the flea on a rat that appeared in Europe from Asia, having come from the steppes.  The Black Death came in three forms:  bubonic, pneumonic, and septicemic, all caused by a bacterium called Yersinia pestis.    These three forms had a mortality rate of 30-75%, 90-95%, and 100% respectively. http://www.insecta-inspecta.com/fleas/bdeath/Black.html Skip Knox writes:  ‘The Black Death erupted in the Gobi Desert in the late 1320s. No one really knows why. The plague bacillus was alive and active long before that; indeed Europe itself had suffered an epidemic in the 6th century. But the disease had lain relatively dormant in the succeeding centuries. We know that the climate of Earth began to cool in the 14th century, and perhaps this so-called little Ice Age had something to do with it.  Whatever Read more…