To me a zombie would be a body that has gone beyond a living state (i.e. the cerebrum has degraded beyond use and rigor mortiss has set in), but the brain centre, or medulla oblongata, still functions somehow, and thusly the body is on "auto-pilot", knowing basic functions, i.e. feeding, despite having a useless digestive system. It could be possible, in films it's usually a virus, which could in theory infect the medulla oblongata and create enough bio-electricity to spark it back into working. Though no such virus has ever been so sophisticated as to mimic the nerve impulses of any multi-cellular organism, let alone one as complicated as a human.