Land of the Dead/WMG

The zombifying effect is actually a very slow regenerating effect

 * The series never reveals what's causing the dead to rise, with in-universe theories ranging from a crashed Venus probe to nuclear radiation to a virus, and the characters all assume that the zombies are reanimated monsters who will gradually rot away over time. But we see in both Dawn of the Dead and Day of the Dead that they're actually getting smarter, acting more human and remembering more of their human lives over time, which doesn't make sense if the brain's decomposing. The smarter ones even look more human, despite having been dead the longest. One way this could nake sense is if the zombifying effect's actually a slow regeneration process. The effect causes the dead to quickly return to animated life, but their brains are still damaged and fueled by rage and hunger, and their bodies are still stiff and decayed, which causes the Zombie Apocalypse. As years pass and the regeneration continues, though, their brains' higher functions are gradually restored while their bodies likewise begin to slowly repair themselves. Once they've finished regenerating, the dead might end up really being fully resurrected human beings.