I wish to clarify some ideas about zombie physiology. Any plan which I put up will be dealing with either "Dawn of the Dead Remake" or "28 Days Later" zombies, for reasons I shall soon explain.

Suppose, for a minute, that the zombie apocalypse has begun. If the cause was a virus, for example "Rage," then the zeds would not have actually "died" in order to become horrific, man-eating monsters. They are diseased homo sapiens, and as such, function as any biological organism must, in order to remain alive.

These types of zombies DO need food to remain alive. One prediction for their lifespan without food was given in the sequel, "28 Weeks Later," where they stated in the intro that all the zeds had died off by about the 11th week (4 months-ish).
BIOLOGICAL ENTITIES need a supply of base matter to function. Nutrients must be transported through the blood, basic food substances must be broken down and converted into ATP in order to provide energy for bodily functions, and it is highly likely that these zed will still need to breathe oxygen.
Lastly, due to their biological nature, there is no plausible reason for WHY NOT these zombies would not be able to run at full sprint, climb over easily-scalable surfaces, and have rudimentary thought processes, enabling them to kill threats to themselves, without necessarily eating them. (see the movies). ONE OTHER IMPLICATION of biological zombies is that there is a strong likelihood that they would NOT be able to do things that they could not while human: e.g. telepathy, super-strong sense of smell, nightvision, super strength. They could have better smell and vision, but it's unlikely that it would be so significant of an improvement that they would be able to locate survivors from miles away. As for super strength, these zombies can only be as strong as their bodies were when alive. The difference in perceptible strength comes from the fact that they surpass the pain threshold when they are motivated enough. A zed may punch with enough force to break his arm, or run so fast that his muscles detach from his legs. Also, biological zed can not move if they do not have energy, or if their muscles are not functioning. If their quad has been severed, they won't be able to move their legs forward and would therefore stumble and fall.

ZOMBIE TYPE # 2. Possession

If the zombies you face are the legit, until-recently-deceased kind, then biological rules go out the window. There's no scientifically compelling argument for these zombies "starving," and they may not be as easily dispatched. Whereas biological zombies have a major PRO for the survivors in that they may be killed as easily as any other living creature, undead zombies have no reason to follow those rules. Hence, it is very likely that headshots are the only way to kill them.

I'm going to step back for a minute. If the zombies are undead, then yes, it is ALSO a possibility that they may be completely impossible to kill. Well, if that happens, God help us all. There is no hope. The only plausible strategy is to run, fortify, and wait it out.

The thing about Dawn of the Dead (2004) zombies is that they don't need a circulatory system to function. In the very beginning of the movie, the man is bit on the neck and he ---completely bleeds out---. Keep in mind that your lungs bring oxygen to your body, but it gets to your muscles via the bloodstream. What can you not do if you have no blood in you? Transport oxygen, and therefore, your muscles cannot work. Thus, these zed are not biological, but possessed. They are being animated, possibly by "themselves," but there is no scientific basis that can explain it.

A lot of people on this site tend to believe that undead zombies would be slow. I ask you, "why?" If these things are brought back from the dead, and are therefore controlled by Satan and his demons, why on earth do you think that Beelzebub would go EASY on us? If he can make them be slow or fast at will, why not fast? They clearly don't run out of energy, as some zombie plans have postulated. If they're possessed, then things such as: "rigor mortis," "lack of energy," "rotting limbs (to an extent)," etc WILL NOT STOP THEM. They're not biological--they don't have to follow those rules.

Therefore, I am convinced that whether zombies are undead/possessed, or biological, they will be fast. Situations in which they WOULD be slow are: (for undead): several years after infection, rotting finally begins to have an effect as their limbs drop off, more likely in humid environments, whether cold or hot. If they don't have legs or arms, they can't do much. (for biological): when they DO run out of energy and nutrients due to a lack of sustenance, then they will starve just as any biological creature would. (see the very end of "28 Days Later.") If they can't work their muscles, they'll fall to the ground, twitch, and have numerous problems controlling their limbs. Eventually they would die. And they would die, b/c they hadn't died yet, just been infected.

I must share this bit of knowledge I gleaned from DotD-04: At one point they burn some zombies. Apparently they CAN be killed by fire. This led me to postulate a theory about the undead variety. Post your comments about this:
1. Brains are part of the CNS (central nervous system)
2. If you shoot a zombie in the head, or deliver massive head trauma, said zombie will re-die.
3. Burning zombies also kills them.
4. Nerves exist everywhere in your skin. You also have nerves going to your muscles, which is how your brain tells your muscles to move.
5. COMPILED THEORY: Massive nervous system damage, whether to a high percentage of the body's nerves, or to the brain, will kill an undead zombie.

The implications of this theory are that you might still have a chance, even if you're low on bullets. To quote Justindude, "the only time you can possibly have too much gasoline is when you're on fire."

SUMMARY OF THE DIFFERENCES BETWEEN THE TWO MOST LIKELY KIND OF ZOMBIES:
Undead:
1. Not able to be killed unless by destroying the brain or delivering MASSIVE amounts of body damage.
2. Are just as strong and fast as biological zombies, except they never stop.
3. The only way for an undead zombie to die of 'natural causes' is to decay, over a period of 5-20+ years.
4. The only way to become a zombie is to be bitten.
***Short explanation: since this is not a biological disease, "infection" might not have to occur due to a contaminating agent. You may very well be safe even if you get zombie blood/spit/piss in your mouth. Because these are zombies from Hell, it's very likely that an arbitrary rule, such as only being turned if you're bitten, exists.
5. May or may not have extraordinary senses. This bears examination before attempting complex maneuvers.

Biological:
1. Able to be killed easily with similar amount of effort required to kill normal humans.
2. Most likely highly infectious, stay clear at all times. Ranged weapons are MUCH BETTER than melee weapons with this kind of zombie.
3. Zombie lifespan may be significantly more shortlived than the undead variety. "Holdout" plans should be effective. As long as you don't alert zombies to your presence, you should be able to survive until they die out.
4. Due to their infectiousness, the disease may be airborne (you're fucked, unless you're immune, but be cautious of other survivors in your group), or able to be contracted through bodily fluids such as saliva, blood, bile, seminal fluids, urine, feces, and possibly even sweat.
5. Zombies will likely not have extreme improvements of various senses. There is a possibility that the zombies may be relatively intelligent, however. Zombies may be able to climb trees or fences (actually, the undead in DotD-04 could do this, too, so the best defense is to be high up with the only way up being to scale a sheer brick/rock/metal face, which should deter any zombie that hasn't evolved velcro grip, or something else freaky like that)

These types of zombies have no reason to be slow. They should be treated, in theory, as if they could be fast, so as to be better prepared. Plans involving moving on foot or on bikes are strongly discouraged, as these zombies should be able to run at speeds up to 25-30 mph. (Strong pedaling of a bike brings me up to 15-22 mph).

If the zed turn out to be slow, you're lucky! Have at 'em. No need for holdout plans.
If the zed are fast, hole up somewhere high up, with tons of food, and tons of guns. (literally: +2,000 lbs)

I hope this makes sense to everyone. I don't see any reason why the zombies wouldn't be fast, so I think it's a bad idea to expect it. Prepare for fast zombies, and you'll already be set to deal with the slow ones.

--one final thing: the behavior of the zed in both movies was as such: both types shambled slowly and aimlessly if they could not locate humans. The only times that they moved fast was when it made sense, e.g. if they saw a human.
There are two rules for zeds upgrading from shuffling to more aggressive behavior:
1: the human must be on the same plane as the zombie. Being high above them confuses them if they can't figure out how to get to you.
2: there must be nothing in the sense of a barrier between the human and the zombie. If there's shatterproof glass (may not hold out 2,000 zombies, but it'll hold out 2-3 at least), then the zombie will slap against it and moan, wondering why it can't get to the human.

Keep these things in mind, and you can avoid the awful adrenaline surge you'll get when you're chased by zombies in the open.

I hope this wasn't too long. I may have been a little bit repetitive, but hopefully I covered everything. If anybody can find any flaws with this reasoning, please point them out. But personally, I think it's sound.

My plan will involve being very high up, with lots of food, and tons of .22 ammo for casual sniping.