I really hope I get better ideas for blogs soon. But if you are taking me seriously you should realize I have nearly zero clues on what I am talking about and do not think the zombie apocalypse could happen. Enjoy.
Brain parasites are a possibility, they change how animals work just to live, an example of one that effects humans is toxoplasomosis, it basically makes humans love cats more. Say a brain parasite could only breed though humans only it could make them cannibalistic, and control then to the point they are brain dead. Also if scientist tried to manipulate a brain parasite and somehow messed up that could jump start it.