Let's pretend for a moment that zombies are real (as if half of you weren't already daydreaming about that very thing). Have you noticed how most zombie movies take place only after the apocalypse is in full-swing? By the time we join our survivors, the military and government are already wiped out, and none of the streets are safe.
There's a reason the movie starts there, and not earlier. It's because the early part, where we go from one zombie to millions, doesn't make any sense. If you let the creeping buzzkill of logic into the zombie party, you realize the zombies would all be re-dead long before you even got a chance to fire up that chainsaw motorcycle you've been working on. Why?
#7. They Have Too Many Natural Predators
Do you know why we, as humans, are at the top of the current food chain? Not because we're hard to kill (well, with the exception of Steven Seagal). We're not; we're little more than tasty flesh bags waiting for an errant horn or claw to spill our guts like a meat pinata. No, we're on top simply because we are so absurdly good at killing things ourselves. A good offense, as they say, is the best de-LOOK THERE'S A DUCK! MURDER IT!
We are simply too smart and too well-armed for any wild animal to hunt. Now consider the poor zombie. It lacks every single advantage that has kept humanity from being eaten to extinction. It wanders around in the open, it can't use weapons, it can't think or use strategy. It doesn't even have the sense of self preservation to run and hide when it's in danger. And, it's made entirely out of food. It's easy prey for any animal that wants it.
If you're saying, "Sure, but it's not like my city is full of bears that can come eat all the zombies," you need to think smaller. Insects are a major pain in the ass for living humans, and in some cases, being able to swat away flies and having an immune system is the only thing keeping us from having our eyes and tongues eaten out by maggots. Zombies in any part of the world with a fly problem are going to be swarming with maggots in short order, meaning that most of their soft tissues will be infested, and their eyes will be very quickly useless.
Not so disgusting now, are they? OK, yeah, but show a little respect.
We'll scale up a bit: In America alone, we have bears, wolves, coyotes and cougars, all of which can put well-armed, thinking, fast-moving humans on the menu, if the conditions are right. To most predators, the "right conditions" are when the animal is weak or infirm, or otherwise generally unable to defend themselves, like a walking corpse. Hell, just think of the millions of stray dogs out there who'll quickly learn that zombies are an easy meal.
Now imagine zombie hordes wandering Africa. Between lions and cape buffalo (and hippos, and rhinos, and elephants), we'd finally have a disease that Africa is better suited than the rest of the world to defend itself against.
#6. They Can't Take the Heat
It's generally accepted by zombie experts that they're going to continue to rot, even as they shamble around the streets. What the movies fail to convey, however, is the gruesome yet strangely hilarious effect the hot sun has on a rotting corpse.
The first concern is putrefaction. Thanks to the plethora of bacteria we use in our colon for digesting plant matter, called gut flora, our bodies are ripe for decay the second our heart stops. Since heat speeds the growth of bacteria (which are plenty happy to start feasting on you once your immune system is no longer a concern) the zombie's got a looming expiration date the very second it turns.
Dead bodies bloat because of the gases created by the bacteria, meaning that in warmer areas even Abercrombie Zombies are going to start getting fat in the first few days. After a few weeks of this, the nasty, bloated zombie army is going to start doing something that is simultaneously the most awesome and disturbing thing a zombie can do: they will start exploding (CAUTION! Pictures!). The warm, moist conditions in the tropical and subtropical parts of the world (or even just summer in the temperate parts) speeds this condition, meaning a July zombie outbreak pretty much anywhere would be over in a few weeks just by virtue of the rampaging monsters bursting like rancid meat balloons.
At the other end of the heat spectrum is dry heat. If you're in Phoenix or the Sahara when the apocalypse hits, the zombies might begin to mummify in the blazing sun and heat. While the normal symptoms of dehydration are not a concern for a zombie, there is the problem of desiccation. With no reasonable means of replenishing the water in their cells, zombies walking around in the Texas heat all day are going to suffer cell damage due to direct sun exposure to their skin, and thanks to the drying effect wind has, the Southwestern dead will stumble around more and more ineffectively until, at some point, they simply drop and wait for the scavengers to come pick them up for the annual Slim Jim harvest.
So they'd better hope the outbreak happens during the winter, right? Well...
#5. They Can't Handle the Cold
Zombies are dead meat. No arguing that; it's their one defining characteristic. But everybody focuses on that "dead" part like it's such a huge deal. They often forget about the "meat." Do you know what else is dead meat? Steak, hamburger, possibly even that red grease mush inside of Taco Bell food.
Look at it.
When flesh is alive, it's got all sorts of defense systems to keep it that way. When it's dead, you have to throw it away in about a week even if you seal it up in plastic and keep it at a carefully modulated temperature. Now, your first inclination may be to think of cold as dead meat's friend, after all, the surest way to defeat that week-long deadline is to freeze steak, keeping it fresh for months. But don't forget: Unregulated cold does awful shit to formerly living things. If you live far enough north, the zombie apocalypse will probably work itself out the first time it tries to go outside. The first zombie-killer is the simple fact that the human body is mostly water, and water freezes. Once the temperature drops to freezing (or near it with a high wind chill), zombies will become significantly more rigid.
No word on them transforming into snow monsters.
After enough exposure, a dead body is going to be frozen solid and not chasing down any screaming victims, no matter how delicious and Rascal Scooter-bound they might be. It's also safe to assume that zombies wandering around in a wintry wonderland are not going to be wrapped air-tight in plastic like we do with food, so freezer burn becomes an issue. Seriously. The same thing that ruins your ice cream also ruins the Undead Onslaught. The freezing of the flesh at night, combined with partial thaw during warmer days, then refreezing again sets up the perfect conditions for the onset of freezer burn, which results in the cells dehydrating as water evaporates, even when frozen solid. Freezer burned meat isn't just dead, it's destroyed.
#4. Biting is a Terrible Way to Spread a Disease
Hey, remember that time when that dog got rabies, and then a day later, every single other dog on the continent had it, except for a small band of survivors huddled in a basement? No? That never happened?
Nearly all of the zombie movies agree on one thing: They reproduce like a disease, one that spreads via a bite from the infected (like they have a virus carried by zombie saliva or whatever). But this also means their spread should be subject to the same rules of a normal epidemic, and biting is a shitty way to get an epidemic going.
The successful diseases have some really clever way to invisibly spread from victim to victim. The flu has killed tens of millions because it floats right through the air, the black plague was spread by fleas, etc. Not a single one of them requires the infected to get within biting distance to spread their infection. Sure, sexually transmitted diseases like AIDS work that way, but that's only because the infected can pass for the uninfected. Nobody is going to be having sex with a zombie.
Though Google Image Search does turn up a large volume of zombie porn
But let's say there is an outbreak, like if one zombie was able to bite 30 people in the crowd at an Insane Clown Posse concert before they figured out it wasn't part of the show. It's not like mankind is just utterly confused about what to do when an infection breaks out. In America you have the Center for Disease Control (CDC,) who don't tend to fuck around. Seriously, it's on their business cards.
Remember the SARS outbreak? That originated in China. The CDC and the World Health Organization put the clamps down on international travel the second it was found to have spread to North America. Flights were grounded, travel between borders was locked tight and only 43 people on the entire continent died.
No one was overlooked.
With zombieism, they don't even have to solve the mystery about how it's transmitted. It's that guy biting people. Shoot him in the head.