His soul was the last to go.
A young man, while out on a field trip, gets bitten by an ill looking person. When he comes back home he starts getting sick and a slow but steady degeneration begins in his body. He is turning into a zombie. He leaves his girlfriend and moves into his own place. In order to survive he needs to eat human flesh. Against his own consciense, he kills people and eats them in his living room. But things start getting worse as his degeneration progresses.