There is 1 zombie to start out, the rest are the Allies.
The allies have loaded guns.
The zombie goes and tries to kill people. Once it kills someone, that person becomes a zombie. The zombies try and get rid of all of the people.
The people can only see 10 feet around them, so that makes it really hard.
It's fun stuff.
Funny story: I was playing in Caen France, and there's a rooftop that's impossible to get to except jumping by fence. So the allies (with guns) hid up there. All the zombies had to line up on the fence and get shot at and try and jump. So we thought they ran out of ammo and we all piled onto the fence, and we started to beat each other to get first to jump. And then the 3 allies opened fire on us, killing all of us. It was so funny.