If you listen to the radios in der reise from world at war, it explains how the zombies came about. ill explain the jest of it.

A meteor fell on the earth containing element 115. the germans experimented with this element to create weapons and machines, a few being the ray gun and wonderwaffle. they also used it to make the teleporters.
a scientist first started by trying to teleport soldiers in his machine. he was unsuccessful. during the radio playbacks, he buys his daughter a pet dog, who happens to be pregnant. during one of his experiments, he ties the dog to a table and teleports her. the dog vanished, but didnt reappear. soldiers who were successfully teleported became unresponsive, and prone to erratic behavior. in a sense, they became the "zombies" you fight.
when the lab was overrun by zombies, the scientist and his daughter used the machine to teleport together, where they both vanished, and became that weird voice you hear in der reise.

thats the basic story outline. i may have gotten a few things slightly wrong, but thats how it went for the most part.