Mitochondrial Transmortem Mutagenesis

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 physiology that causes the dead to reanimate and attack the living.

A few minutes after somatic death, the mitochondria mutates into special organelles which produce a new form of energy to power the motor neurons and lower brain functions. As the body becomes self-sufficient on this energy, the respiratory and circulatory systems become vestigial, ending the body's dependence on oxygen. In the process, all higher brain functions are destroyed. This leaves only the cerebrum to govern behavior. That in turn creates very animalistic behavior. Necrosis of the hypothalamus produces a massive flood of neurotransmitters, enzymes, and hormones such as norepinephrine and dopamine. These effects, combined with the painful symptoms of body-wide tissue decay, induce a psychotic rage and persistent hunger.

Although their cognitive functions are almost non-existent, their residual natural human instincts causes them to disregard cold putrid flesh similar to their own as a food source. Likewise, they are instinctively attracted to warm healthy flesh. An augmented sense of smell enables them to easily distinguish the two. The body-wide necrosis added with the cessation of immune system activity, leads to the body being flooded with lethal bacteria. That makes the bodily fluids highly toxic. Exposure to a sufficient amount of their saliva for example, will kill a person in a matter of hours. Severing the top of the spinal column or massive trauma to the brain are the most effective methods of killing them.