The Black Death originally came from the Gobi Desert in the late 1320s. No one really knows why. The first sickening plague was the plague of 1347 to 1350 though out Asia and Europe. This came from central Asia probably carried by fleas residing on the black rats. Plague first reached the shores of Europe when it got to Italy in the Spring of 1348 though the black rat that were onboard merchant ships unleashing death across all of Europe.   

Black rat

Black rat at the origins of the black death