I've recently been reading the Day by Day Armageddon series by J.L. Bourne.
What makes the series better than most average zombie books is that it doesn't try to explain in depth how the world quickly fell to the zombies. With scenarios like this; it's best to incorporate that into the initial hand-wave that creates the zombie virus or whatever it is that makes the dead come to life and hunger for the flesh of the living.
In the first book in Day by Day, you get snapshots of the slowly deteriorating situation in the world – you find out that an infected person was brought to the CDC in Atlanta; and then later learn that there's an outbreak in Atlanta. The writer doesn't attempt to explain how someone in a sealed building with many safeguards can somehow escape to infect the city; and thus doesn't look stupid.
This is where World War Z by Max Brooks failed completely utterly. He tried to explain in detail how the Zombies took over the world – and constructed increasingly infantile scenarios that required the world's militaries and police forces to take large amounts of Stupid Pills™ and Act of Plot Pills™ in order to get Zombiepocalypse.
Likewise, in Plague of the Dead by Z.A. Recht, you see this problem.
In Plague, the initial zombie virus outbreak occurs in Africa, which is plausible -- it then spreads through air travel to other parts of Africa and the rest of the world; but for a good while, the majority of the zombie plague is in Africa.
All good and dandy?
So Recht has the world's militaries quickly set up a qua-r-an-tine around Africa, with naval vessels sinking ships that try to break the quarantine, and the US gets the job of ensuing that nobody jumps from the African continent to the Eurasian continent via land.
So what they do is setup a quarantine line along the Suez canal. Recht makes explicit references to Combat Engineers blowing up or flooding the bridges or tunnels that go across the Canal.
In fact, the last rail bridge across the Canal being blown up by Engineers is shown “on screen.”
They then lay out multiple lines of defense; sandbags and barbed wire on the Canal banks as line one, move back some more, I forget what the second line was, but the third and last line was tanks and other AFVs.
It's also mentioned explicitly that the Ronald Reagan CVBG is nearby providing qua-r-an-tine via sinking ships with F-18Es.
All of you with me so far?
OK. So the zombified inhabitants of Cairo (it's stated that there are tens of thousands of zomboids in the swarm) slowly approach the Suez Quarantine line, following some hapless driver in a truck who is trying to flee the zombies.
The truck driver gets blown up by Apaches of course; and you see some off screen rocket fire and chaingun fire from the Apaches before they return to base Winchester (all ammo expended) from the other side of the Suez.
So the Zomboids reach the Suez, and the guys in the first line start to shoot at the zombies with M-16s and other small arms. They start knocking off the zombies, etc etc.
The zombies start to actually cross the Canal via Zombie body bridges -- as in they start building a corpse bridge across the Suez.
It's only at about that point that the military realizes:
“Hey, you know what, we have artillery, why don't we start using it?”
So they start shelling the mass of zombies.
Of course, by this time it's too late, and the zomboids start crossing the Suez and biting the faces off the soldiers in the first defensive line. Things quickly unravel after that point, with a mass retreat called, and of course even more biting off of faces occurs.
The big problem is that logically, if you have giant hordes of zombies tens of thousands strong, then they're perfect targets for high powered weapons like napalm, cluster bombs, fuel air explosives, artillery, even nuclear weapons. So why not use them?
One of the biggest problems with area effect weapons against humans is that humans are smart and will seek cover after the first weapons strike; meaning you kill the most amount of people in the first few strikes, and then increasingly less as people take cover. With zombies, this is not a problem, you could just keep dropping FAEs into a zombie horde and killing scores with each drop.
Here’s a simple case to put modern firepower into perspective for you non-military-inclined readers:
A single B-1B “Bone” can deliver thirty CBU-87s. These are 1,000 pound cluster bombs which have been retrofitted with GPS guidance, creating Wind Corrected Munitions Dispensers (WCMD).
Each CBU-87 can basically cover an area of about 400 by 800 feet in bomblets, or roughly 320,000 ft2 (29,729 m2) per CBU.
Some numbers for crowd densities are:
10 ft2 per person (each person is an arm’s length from another
Tight Crowds: 4.5 ft2 per person (each person is less than an arm’s length from another person)
Mob Crowds: 2.5 ft2 per person (no space between people)
You can see from this that if you dropped a single CBU-87 onto a “loose crowd”, it would have a very good chance of maiming about 32,000~ zombies.
Due to the CBU-87 being a GPS guided weapon, a single B-1B could then maim 960,000 zombies in a single pass; because each cluster bomb could be guided to an individual aimpoint for maximum footprint coverage.
Back in 2013 the USAF had about 60 B-1Bs on active duty (30 x CBU-87s each), 20 B-2As (34 x CBU-87s), and 240~ A-10s (7 x CBU-87s). This is a brief sample of what in the USAF inventory can carry them; but those four platforms alone could deliver 4,160 CBUs in a single sortie; potentially destroying upwards of 133~ million zombies.
Additionally complicating matters is that according to “conventional” zombie lore, they can only ‘reproduce’ via infecting humans or via human deaths; so their numbers have a hard upper limit placed on them.
If a human can kill five zombies at a minimum before he succumbs (not a bad estimate for someone armed with a blunt weapon like a baseball bat); and the city in which this is taking contains 50,000 human survivors and 75,000 zombies; then by the time human numbers have fallen to 31,250; the zombies have been eliminated, for a cost of 18,750 humans.
If we increase the kill ratio to 20 zombies before death (firearm equipped survivor on top of a roof plinking away); then only 3,948 humans out of the 50,000 die before the zombie menace is eliminated.
As you can see; Zombies are a threat from the stone age up to when repeating firearms (revolvers and the like) were popularized in the 1840s onwards. Yes, I know you can use a bow and arrow or a slingshot to kill zombies; but those two weapons require a lot of training to hit the target reliably, while the repeating firearm is much simpler.
Additionally, some nations are just less susceptible to Zombiepocalypse than others – in the United States, there's just too many guns floating around for Zombies to ever become a serious national threat, though scattered pockets of zombies would still be a problem.
Ruminating on the problem further, I've concluded that for a Zombiepocalypse to have a good go at causing mass breakdown, the following things really help:
Everyone is Infected
The concept behind this is that 95% of human beings on Earth are already carrying the zombie virus in dormant form. It could even be possible that humanity has always had the zombie virus in their DNA – it's just been in that “trash” DNA that we don't use...until a passing radioactive alien meteorite swarm™ activates that bit of DNA.
At first, we wouldn't notice it; since a working immune system is more than strong enough to keep the zombie DNA down. But if you die, this means that there is a time interval for the zombie virus to take over your body before cellular death becomes too irreversible to “reanimate” your corpse.
Mass outbreaks occurring all over the world at the same time which cannot be easily contained also become a lot easier to handle plotwise through this idea.
You already have a ready-made portion of the population that's susceptible to zombification because their immune systems are weakened through either sickness, malnutrition, or drug use. This also means that if someone gets sick enough to distract the immune system, that person would go zombie; meaning that disease outbreaks would be followed by zombie outbreaks; and you would have to make sure you don't cut yourself too badly or consume undercooked meat.
Non-Traditional Threat Vectors
Human zombies can be dealt with fairly easily. What about the rodents who live in your walls? Or the stray cats? It's within reason that the zombie virus also affects the other members of the mammal family. So you could take extreme measures to board up your house to avoid the packs of zombies in the streets, only to be bit in your sleep by a zombified rat.
If the zombie disease is spread by contaminated blood; then mosquitoes could be a major vector; they could suck the blood of a zombie, and then bite a non-infected human. This would be a significant problem in malarial areas.
If you make the vector that kicks off zombification airborne; it helps a lot, since people would now have to wear gas masks to avoid being zombified.. However, this has a very strong chance of setting off the reader's “bullshit” alarms; because we've already hand-waved a lot of stuff to create zombies that rise from the dead and follow the traditional zombie tropes, and this last hand-wave may be the thing that pushes the reader over the line.
Less-Than-Lethal Infection and Extended Gestation
With a 100% turn rate and infected people turning in a time range of a few hours to a day...there is no way you would be able to have Zombies take over even the most rudimentary outpost; since anyone who was bit would show right away and would be killed immediately.
If there's a 75% or above chance of survival from a zombie bite; and they don't turn into a slavering zombie until a week has passed, this means people will be more reluctant to kill family members, close friends, etc. Also, a time lag of a week or so means that people can be infected and still get into secure areas.
Semi-Smart and Semi-Agile Zombies
I'm not asking for a zombie that retains 100% human intelligence after conversion; but something on the same scale as a relatively stupid monkey. Monkeys can figure out after much trial and error how to open cages which are only secured, not locked. That, and monkeys can climb things.
Being faced with zombies who can slowly scoot up a tree or a drain pipe means you can no longer easily retreat to a roof and gain absolute safety. Likewise; simply closing a door on zombies would no longer work – you would have to lock the door or block it.
Naturally, “fast zombies” would be a subset of this threat. It would also be likely that this type of threat is limited to those infected corpses which are “fresh”, before heavy decay has set in.