It's Thanksgiving today; for some, it's a day of eating with abandon and otherwise moving very little, if at all. In a way, that makes it a great day to be a zombie. Of course, for those who celebrate the holiday the meals will be special things you don't get every day - that's just one thing that makes being alive wonderful, because once you're a zombie it's leftover brains and innards all day every day until you rot away.
It's an important subject, so let's go over one more time the ways in which we might kill a zombie.
Zombies need, at the very least, part of the brain. No matter what might animate a zombie corpse, the brain is the only thing that can bring the body into order and have it function properly, running and chasing and all that. that's why they need the brain, and that's why we need to destroy it. There's not much chance of knowing exactly what parts of the brain a zombie needs, so just shooting it with an arrow or a small bullet might not to the damage you need; be as thorough as possible when it comes to killing a zombie, because even a little bit of life is too much to allow the dead.
Maiming a zombie will never kill it. All you can accomplish with broken or severed limbs is making it harder for the zombie to kill you. That's not a final solution by any means, but still a perfectly good thing to do. Broken bones won't support weight, torn muscles won't flex with any real strength or at all. Nothing can heal, either, without real life blood and proper body functions, which the living dead won't have. Zombies may or may not rot while actively "zombified", so to speak, but none the less their body condition will continually degenerate from failure to properly heal. Eventually, any surviving humans will be able to kill off any remaining zombies.
Or will they?
Because looking at these simple facts, we can deduce that there are many things that will not only fail to kill a zombie, they can actually help the zombie survive much longer.
Living in the desert will not kill a zombie; heat stroke might cause brain damage, but you can't count on it disabling the zombie, much less killing it - and it might not even happen, since zombie physiology is so different from ours. Worse, though, the arid climate will keep the body from rotting, preserving it like beef jerky - or a mummy, if you like. Maybe those toilet-paper-clad folks with the stiff arms weren't ancient kings, but restless dead that the Egyptians wanted to keep tucked away in a really heavy box. And so we let them out... good call... luckily, those crafty Egyptians knew to take out the brain, and probably even remembered to do it most of the time.
On the opposite end, the arctic tundra is equally favorable to the zombie lifestyle. They feel no cold, don't get hypothermia; they'll probably run just as fast there as anywhere else, excluding the effect of running through snow. You, on the other hand, will be slower, giving the territorial advantage to the zombie. Like the dry climate, the cold will prevent rotting. But since zombies don't regulate their body temperature, it's possible for them to actually freeze solid. This could work out in two different ways.
Scenario 1: the ice crystals pierce a lot of brain cells; the zombie thaws, and all its head is full of mush that's not capable of sending a single nerve impulse. Freeze and thaw a nice ripe banana to see this for yourself; great for making ice cream, not so great for being a banana anymore.
Scenario 2: the ice crystals pierce some brain cells, but not enough of them to completely still the zombie; this is more likely in an arid climate, since the zombie might have lost a lot of moisture before freezing, thus reducing the amount of ice and the associated damage it can cause. When this zombie thaws, it's still a zombie, and it might want to kill you. It'll probably be a good bit less functional than it was before the freezing, but if it can swing its arm or close its jaws, it's a dangerous creature and should be killed.
Being underwater probably will not kill a zombie. Zombies don't need air, so they can't drown. Sharks might sense them as edible meat, and who can say whether the plague will transfer to them; I hope not, because the thought of an undead shark is just too terrifying to me.
If they don't get eaten, zombies could theoretically survive underwater for a long time. If they're buoyant, they'll float and drift with the water currents (I assume that swimming, an iffy prospect for many living people, will be well beyond most zombies) and they'll eventually wash on shore somewhere; if they sink, they can walk along the bottom. The good news there is that the ocean floor is incredibly vast, and the walk up the continental shelf would be pretty steep, so getting back out of the ocean would be a problem. But that's also kind of a problem - if there are zombies in the ocean, can we ever be rid of them? We certainly can't dive down looking for them; there's simply too much area to search. This answer will depend at least in part on whether they rot or not, which is the subject for another post at another time. The watery environment would be very conducive to rotting, but if they fail to rot naturally they could be down there indefinitely.
Now, join my horror story from this point. Imagine if you will that a zombie was wandering about on a glacial plain in Alaska, and froze solid during the night. Never thawing enough to move away, the zombie was slowly covered with snow, layer after layer, and became part of the glacier.
Some hundreds of years later, the zombie breaks off with a piece of the glacier into the Pacific ocean. By this time, all the zombies on land have been killed by surviving humans, and society is beginning to rebuild.
The iceberg melts, and the zombie sinks to the ocean floor. The sharks don't want him, the hagfish don't want him, even the bacteria in the water don't want him - his flesh is so undesirable to the world, it's left to fail to rot. So our zombie wanders about, no thoughts to guide him, no human scents to lure him.
Until, some indistinct time later, a fisherman pulls him up in a shrimp net. This shrimp net didn't have the special door that turtles can use to escape, so he gets pulled up with the catch. And hooray, live humans. He's a happy zombie again, like he hasn't been in centuries, and now he's got happy zombie buddies to hang out on the boat with.
Sooner or later, the boat meets up with land, where perhaps a thousand years or more could have passed since the zombie apocalypse; do people remember how to deal with zombies? did records survive that dark time? Do the children of they day think that it's all a big myth? Well, no matter what the state of the world by then, the zombie scourge has fallen upon the world again; it's doubtful that it will fare all that much better this time than it did the time before.
It seems possible then that zombie outbreaks could come in cycles, at least until the world is destroyed by comets or nuclear holocaust. Zombies are more survivable than those other two, but impossible to be done with permanently. Have you considered the possibility that one has already occurred? I remember seeing on TV a while back some researchers doing mitochondrial DNA analysis, and they came to the conclusion that once a very long time ago, before any recorded history, there was a huge disaster that killed most of the world's people, leaving only a small number to repopulate the world. They connected this to the story of the Biblical flood, but to my mind it could just as easily be an ancient zombie plague. Perhaps it was both, and some zombie is stuck deep underwater, maybe even buried in silt, but still very much a zombie. Should this being ever be brought to the surface, the plague will begin again.
And on that cheery thought, I'll let you get back to your pie and leftover turkey. Next week we'll have a little seasonal advice, on how to modify your zombie survival plans to suit the oncoming winter.