IIRC there was a pacific island mentioned in the ZSG that was used for death rituals by natives, as witnessed by an explorers account. The dying would be left on the island and the Zombies would come and eat.
Well the ZSG strongly implies (pretty much outright states if my memory is correct) that the Japanese discovered this island during WWII and attempted to create an army of Zombie soldiers.
With their defeat the Communist nations became aware of this and attempted to do the same. After the fall of the USSR only China still actively participated in these experiments, the result being that it caused the outbreak at the stat of WWZ (hence why the Chinese tried to unsuccessfully hush things up at first).