You think all those writers in Hollywood are just making this stuff up? Like they have that kind of imagination? These are the people who have unleashed THREE Alvin and the Chipmunks films onto an unsuspecting public, so I hardly think they have the wherewithal to come up with something like a zombie invasion. (On the other hand, there's The Walking Dead)
As the following Infographic pleasantly illustrates, the Zombie Invasion could happen at any time.
This previous post contains an infographic by the same people: Infographic: The Evolution of the Zombie