The port of Kaffa, and Genoese traders.
And actually, it wasn't the bodies. Once the body dies, the fleas abandon the corpse, and the corpse is effectively rendered non-infective. More likely it was just black rats moving between the Mongol camps and the city during the siege, then stowing away aboard Genoese ships. If it were "use of WMDs", it was hardly intentional on the Mongols' part.
And even if you argue unintentional "use" is still "use"...it's still not the ultimate. The introduction of Old World diseases (mumps, measles, syphillis, smallpox) to the New World was far more devastating.