There are any number of reasons why an attack of George Romero-style zombies couldn't actually happen, from the dogs that would love to eat them some fresh bones to the fact that, well...dead things just don't fucking come back to life and start eating. That's what we call a "conceit."
So if a local TV station tells you that it's actually happening, as one in Great Falls, Montana did - don't believe them. It was almost certainly the work of hackers.
Unless that's just what the powers-that-be want you to think...