There won't ever be a zombie apocalypse -- the only reliable way to turn into a zombie is to listen to my Great-auntie Sondra tell stories about her childhood, and I don't think she could ever get a big enough audience to cause an apocalypse.
There would never be a zombie apocalypse also if there is it would never last for long. In stories zombie's victims turn as a result of bitten by other zombies or if a zombie-creating virus travels by air, sexually or water.
A zombie is a person that was dead but was brought back to life so the zombie apocalypse you are asking about its been happening for a very long time. look at the hospital when ever your heart stops they use a defibulater to bring you back to life thus creating a "Zombie."
Yes, it actually can happen, just not the way you would imagine. People don't believe in it because of the nature of the idea, "Undead Dead". The real reason behind the start of a true apocalypse, probably starts somewhere in the virus field. I have actually researched this and there is a virus in rats that shuts down the cortex of the brain. Along with the cortex, it basically shuts off everything but stem. The stem controls movement and natural instincts, including to feed. Now with the rest of the brain shut down, the rat no longer knows when its full and also no longer knows what isn't food, mainly other rats. If you graduated at least through tenth grade, you know that a rat and human function about 90% the same and our brains run just about the same, just humans are larger in size and have a brain proportional to the size of them, being this, they could contract the disease via scientific experiment and use a test subject to try it. It is what you think, a live zombie, not dead; basically acts the same but isn't dead.