Starting in Italy ,and being spread by rats. The Black plague (or black death) spread all through out Europe and by a period of three years this sickness had managed to wipe out 25 to 50 percent of the European population. The actual disease was carried not by the rat, but the rat's fleas. Which bit people and infected them.
http://www.eyewitnesstohistory.com/plague.htm
http://www.eyewitnesstohistory.com/plague.htm