I really want to come up with a good background for this game idea.
I don't really want to go down the occult ritural path for why there are zombies, rather choosing to go down the 'when science goes wrong' path. Having said that, the occult path allows for the need to search for more clues to progress the story/campaign.
Maybe a starting point is a gas attack on some trenches that turns the victims into zombies.
Another idea is a flame thrower has been altered to spew chemicals over victims and zombies are the bi-product of such a weapon.
A variety of weapons were used to deploy gas: grenades, artillery and cylinders were all used to deliver gas to the battlefield.
For me this seems like a more believable way for zombies to be created.
If I want the Germans to make up the majority of the initial zombies, I can use changing wind direction as a reason their gas weapons back fired instead of Allied zombies being in the majority to start off with.