Kaffa was an African state.
According to the First Doctor, a large epidemic of bubonic plague originated in 1346 when Mongols in Janibeg Khan's army besieged Kaffa. The commander of Kaffa's defenders ordered diseased corpses to be catapulted beyond the city walls. The illness could spread to the Mongols, killing many of them.
Dodo Chaplet feared the idea was possibly influenced by a similar strategy employed against the Mongols at Kiev in the 13th century, and it came to be regarded as the first instance of biological warfare. Soon, Genoese merchants travelled across the Mediterranean, spreading the plague to ports in southern Europe. It moved on to Spain, France, Germany and Britain, and then to Scandinavia and Greenland. One third of the European population died as a result of the plague. (PROSE: Bunker Soldiers)