The zombie apocalypse refers to a large group of mindless subhumans attacking people who have brains. Actually it's already happening. Just look at the media. One of the most realistic representation of this is the wall st protest. 30% of the population claiming to be against the 1% ever the while using the products, clothes, and facilities of those who they claimed to be against.
Zombie movies have been part of popular culture for around 3 decades now. With the advent of video games the idea of a zombie apocalypse has been hyped up even further. While it may be possible for some virus or other mutation to cause something similar, I think it's much more likely the apocalypse will consist of something like an asteroid or solar flare.
Zombie are a fad that wanes and ebbs. Over the last few years, zombies have returned in literature, books, TV, movies, you name it. This is fairly typical of the storytelling medium, but it does not indicate that something like the Zombie Apocalypse is real. The Zombie Apocalypse was unheard of just a few years ago and in a few more it will retain that position.