Mitochondrial Transmortem Mutagenesis is the condition that creates "zombies" in the universe of George A. Romero's Night of the Living Dead and subsequent films of the franchise. Within that universe, a radiation cloud engulfed the Earth and permanently altered human mitochondria in a way that causes the dead to reanimate and attack the living. Massive trauma to the brain is the most effective method of 'truly' killing a zombie.