In your head, in your head, 
zombie, zombie, zom…
Everybody these days has a zombie survival
plan. Mine mostly involves weeping in the
fetal position in the dark? But fortunately
or unfortunately, depending on how much you’ve
already sunk into your zombie contingency
plan, a traditional, undead-zombie outbreak
is never going to happen, because zombies
are physically impossible.
Here are five reasons why.
OK: The conditions of the zombie outbreak
set in. Let’s say a space probe returns
from Venus, explodes in the atmosphere and
that somehow wakes up all the corpses on the
planet. Doesn’t really make sense to me,
but Romero said it, and I believe it, so...
Imagine you’re a corpse, you’re just lying
in your coffin, minding your own business,
when suddenly you wake up with a craving for
the moist savor of human flesh. Or whatever,
you know.
That hunger you feel suggests your body is
still subject to the laws of physics – namely
the conservation of energy. If you’re going
to be walking around, chasing survivors and
moaning for brains, all of that activity costs
your body energy.
And for energy to be going out of your body,
it also needs to be coming back in.
Normally, this happens when a living human body
converts chemical energy from food in the process we
know as metabolism.
But an undead creature like a zombie has essentially
already declared energy bankruptcy. Obviously
corpses don’t have the working digestive
system and metabolism to turn food into usable
energy. A dead body has no metabolism, and
thus it can’t really... do ... anything!
But let’s say we just ignore the fundamental
energy/metabolism problem, and we just grant
that dead, rotting bodies are able to walk
around looking for fresh brains.
We immediately run into another huge obstacle:
the weather. It’s easy to forget that living
organisms spend a huge amount of effort protecting
the insides of our bodies from the whims of
the weather. Just one example: Thermoregulation.
Thermoregulation means being able to keep
your body’s internal temperature within
a certain range, even if the temperature outside
is a good bit higher or lower. Thermoregulation
is a form of homeostasis, which just means
being able to dynamically adjust variables
to keep the internal conditions of your body
relatively stable.
Zombies, having none of these capabilities,
better hope the weather is nice, or it’s
lights out yet again. Without thermoregulation,
sub-freezing temperatures would crystalize
whatever moisture content is left in zombie
flesh, destroying tissues -- including the
all-important and delicious brain tissue.
So: No zombies in winter. Likewise, zombies
in a hot, dry desert climate would quickly
see their moisture evaporate, leaving them
shriveled, desiccated delicious raisin-corpses.
But, let’s say you imagine your zombie outbreak
in very hot, humid, jungle climate.
Everything would be fine, right? Wrong!
Here we get to another problem. 
Have you ever seen what happens if you leave a piece
of raw meat sitting out for a week in a hot,
humid climate, like you do?
That’s right: It’s swarming with flies and smells of rot… that is, if it’s still where you left it when you come
back to check.
Presumably, the same thing would have to happen
to a zombie. Having no immune systems, zombies
would almost immediately become infested with
microscopic opportunists like bacteria and
fungi, not to mention millions of insects
– beetles, flies and maggots, and other
carrion-eating insects. That sounds fun.
These efficient organisms can consume a given
mass of dead flesh at an alarming rate. On
top of that, you can count on vultures, dogs,
raccoons and other adorable large-scale scavengers
to come looking for their taste as well. I
mean, really, every zombie in every zombie
movie should be dragging a train of scavengers
large and small everywhere it goes until its
bones are picked clean.
OK, all right, you’re saying, ignore all
that. Let’s imagine they can walk, moan
and eat brains in a climate-controlled, scavenger-free
zombie wonderland. We’ve pointed out that,
lacking a metabolism, zombies can’t turn
food into usable energy. But there’s another
problem with the brain-eating impulse: If
zombies lack a functioning digestive system,
they probably can’t ... poop.
The brains have nowhere to go, so they would
likely just pile up in the digestive tract
and ferment, which would produce expanding
gases, and eventually the zombie’s stomach
might just burst. That's... disgusting.
Finally, I would just like to point out that
zombies would probably have a really hard
time finding you. True undead zombies exist
in various states of decay, and the soft tissue
of the eyes would be one of the first things
to disintegrate as the zombie rots.
This means that most zombies (except maybe
the really fresh ones) would have no vision,
and thus would have a pretty difficult time
chasing you down. Of course some types of
zombie mythos say that zombies hunt by smell
or hearing. Those senses might not become
impossible as fast as vision, but over time,
the same general ideas apply.
So, old-school, undead zombies? It’s just
never going to happen. But! That doesn’t
necessarily rule out various conditions of
mass psychosis or zombie-like infections.
So maybe you shouldn’t scrap your zombie
survival plan just yet…
You just might need to adapt it to some kind
of mind-stealing murder fungus scenario. Anyway,
what do you think is the most scientifically
plausible zombie scenario? Let us know in
the comments! And if you liked this video,
subscribe. And of course, if you want to know
more about zombie science, check out 10 Reasons
Zombies Are Physically Impossible at Howstuffworks.com!
