I read somewhere that a tiny parasite called Toxoplasma gondii, which lives in the stomachs of cats, could very well be the catalyst for a zombie apocalypse. When rats are infected by this parasite their brains become affected and alters their behavior. Instead of running away from cats they become attracted to the smell of cats and cat urine. The rats run straight towards danger where they can be easily caught and ingested. The ingested Toxoplasma gondii find their way to the cat’s stomach and reproduce to complete the life cycle.
It's said that up to one third of the world's population is estimated to carry a Toxoplasma infection. Just think, if this parasite began to affect humans the same way, infecting the brain and causing them to run into harms way or altering what humans want to eat just for the sake of it's own reproduction! Toxoplasma gondii doesn't just come from cats. It can also be obtained by the ingestion of raw meat.
Now imagine one third of the world's population is carrying a parasite that mutates one day and causes it's carriers to crave raw meat and have complete disregard for bodily injury. I would have to say it would look very much like the Zombie War has begun.
Luckily this is just a theory, for now anyways. Meow!