Let's take a moment to talk about zombies. It was either that or zebras, so bear with me.
We all love a good zombie flick, right? Hordes of the moaning undead ambling about and somehow being able to outrun, outsmart, and outright destroy armies of non-dead, intelligent creatures with the ability to run from danger, plan, think for themselves, and form alliances. Makes sense, right?
And the moral of the story is, without fail: It's not the zombies that destroy you in the end, it's the living you have to watch out for. The moral of this particular story is: It's not the zombies that destroy us in the end, because we're all going to be alive for the zombie apocalypse.
Think for a moment about that one time a raccoon got rabies in, say, Germany, and the next day, every single raccoon in the entirety of the world had rabies. Or, wait, did that not happen? Now think about what you have to do to bite someone. Or, better yet, let's think of what our pet zombie Rob has to do to bite someone. (We all have pet zombies, don't we?) First, he has to find a victim. That's the easy part. It moves. It could just as well be a tree, but let's give him the benefit of the doubt and assume he found a person. Now he has to bend down (hard when you don't have any coordination left, but whatever), get a good grip (also difficult), open his mouth wide enough to fit a nice chuck of flesh in, and bite down hard enough to break through clothing and flesh. Now, assuming he did get a person and not a tree or another zombie, who the hell lets a zombie bite them? All that bending down and opening up and gripping takes time. Our zombie doesn't have time. He's trying to start the apocalypse. Only, he's a zombie, and he's not very good at it, because right now, in the pale moonlight, he just looks like some dude who had many, many homerkins too many down at the local pub. No one is going to let a drunk zombie bite them. No. One. Sorry, Rob, today just isn't your day.
Let's also take a moment to think about the fact that this isn't, say, 1349. It's 2015, and these days, we understand a little bit more about the world. We'd probably treat the zombie apocalypse as a disease, and quarantine appropriately. Yeah, maybe New York City would be screwed, but it's an island. The rest of the world would be safe. (Do... Americans not know this or something?)
Even if the zombie apocalypse started simultaneously in every major city across a continent (in which case it'd be weaponized, and we'd all have much bigger problems to deal with), it still wouldn't get much further than that. North and South America are connected, yes, but the connection is easy enough to defend. Australia is another, albiet smaller, island, and Antartica doesn't count because when does it ever? Worse case scenario is that it starts simultaneously in every major Euroasian city, in which case we'd all die and the Americans would get to rule the world, which is probably something we're better off being undead for in any case.
But back to the point. The zombie virus - let's call it what it is - wouldn't kill its victims. Not at first, anyway. It'd be virus suicide. It'd leave them in a regressed, aggressive state and if it were clever, it wouldn't weaken them at all. So, humankind would be left with a dillema: exterminate these living, breathing, sick people with no knowledge whatsoever of whether or not they're still alive in there, or whether or not a cure could be found, or allow them to run amuck and destroy us. Despite the questionable morality of it all, there's little doubt in my mind as to what we'd do. And that's when the zombie apocalypse gets scary: when a virus leaves us with an impossible decision and forces us to pull the trigger.