I think about it like this... A zombie outbreak via some unknown virus is possible... But for it to be an apocalypse, they'd have to be alive... There are certain sci-fi stories, such as I am Legend, in which the sufferer of the Zombie virus is still alive, can spread the disease, and still be cured. A zombie apocalypse could be possible if multiple infected individuals were kept at a research facility to find a cure, and at least one escaped into a populated area and began infecting others. Then you'd have a population of infected individuals... But as Rex and Sir Red were explaining, humans are smart, we'd quickly realize what's ensuing, and promptly find a way to stop it, whether it's taking out the infected population, or curing them... They'd have to be living so that they would be able to repair themselves and be less vulnerable to the elements, and even then, that might not be the case depending on what other effects the virus has besides neural shutdown and cannibal cravings. In short, Zombies are a possibility, but, they really pose no threat to humanity as a whole. Of course, I'm speaking scientifically... Religiously speaking, if we begin talking about many religious beliefs that believe part of the apocalypse will be the mass rising of the dead, then we may or may not be screwed... But we're not just talking Zombies there, we're talking complete biblical mayhem, Zombies, Natural disasters, and paranormal catastrophe... But that all depends on what you believe in.