Actually scientifically zombies are very possible. This is from the little medical and natural science knowledge I have.
It’s a bacterium that feeds off the bonding structure of the white blood cells, and perfectly lives within the fluids that compound within the nervous system to the brain. However as it converts blood, the human heart rejects it, and stops beating, thus the person dies. Although the bacteria still lives on and manages to have control of the brain’s motor, albeit slow as the blood circulation has halted into a thicker substance, so gone are those quick and calculative reflexes. (Sort of like when someone gets high, as the dope slows down the blood circulation, it increases the person’s stupidity.) Sight in Zombies is limited due to dehydration, thus strengthening hearing and scent senses to seek fresh un-mutated blood that the bacteria hungers. And apparently various bacteria can live in open air as long as their within the slightest moisture, so it doesn’t matter if the body is decaying. And finally, a bullet to the brain amputates the bacteria’s control to the whole body. This could very well be the bacteria’s own next step in evolution.