The first thing Brooks does is set ground rules. Once infected and undead, zombies are essentially monomaniacal brains unmoored from brains' normal contingencies – e.g. a pumping heart, a digestive system, oxygen. Until the brain itself is destroyed, it will stupidly, relentlessly pursue human flesh, using whatever parts of the original body remain at its disposal. Zombies move slowly, with arms – if available – raised toward their target. If a zombie finds prey, it will moan; if a nearby zombie hears a moan, it will move toward the source and let out a moan itself. You see how this could escalate.