What we know:

  1. If you get bitten by a walker but remain intact otherwise, you become a walker
  2. If you die of natural causes, you become a walker (from Season 2, we know everybody is infected)
  3. Walkers are all around since the beginning of the tv show; "population turning time" is fast and not a gradual process.
  4. If you are completely eaten. you do not survive and hence do not become a walker.

Therefore, to justify the spread of the infection to the majority of the population, we have to assume that any walker is a consequence of either a) a bite or b) natural death.

On one hand, chances favor 4 against 1 - that is, I imagine people die when they fight with walkers most of the time, especially in densely populated environments like cities, where most of people/walkers are. Therefore we can rule out 1 as the reason of a population-level spread of infection.

On the other hand, people die of natural causes because of age and diseases (too slow), or for injuries and violence (does not justify why SO MANY walkers are already around and not just a few) or starvation (again, it would be a slow and gradual process).

So, what happened?


Why aren't more people totally devoured?

It isn't explicitly stated on the show or in the comics, but the Max Brooks books World War Z and The Zombie Survival Guide suggest that zombies are very picky, and only eat fresh meat - either still-living, or killed within the last few minutes. This might be true of TWD as well. Furthermore, one zombie can't eat an entire person, even if it tried - you'd have to fit one person-sized amount of meat (minus some volume because large bones can't be eaten - they'd break the zombie's teeth) inside another person's stomach. This is impossible, because a human is much larger than the human stomach, or even the entire human abdomen.

Thus, people can't be totally devoured by one zombie, or even two or three zombies. A person can only be totally consumed by a large group of zombies, and even if there is such a group present, they usually don't finish eating the entire body.

We see this happen in Season 6, Episode 7, when a massive group of walkers attacks Nicholas, but eventually wanders off with much of his body, and his entire face, still intact. Later in the same episode, we see another Alexandrian, who had been trapped by hundreds of zombies, reanimated and mostly intact. This suggests that, as in Brooks' books, TWD zombies will stop eating when the body has been dead for long enough to be no longer "fresh", according to their standards.

Natural Causes:

As we saw in Season 4, natural causes - injury, old age, and disease - are much more deadly now than they were before the world fell apart. There are very few doctors left, access to medicine is extremely limited, there are no hospitals in operation (except rare exceptions like Grady Memorial, which is operating with drastically reduced supplies and resources, and has helped only a handful of people since the world essentially ended two years ago).

Getting sick is now a much more dangerous prospect than it had been when the miracles of modern medicine and technology were available. The survivors are only slightly better off than people were thousands of years ago, when a simple broken leg or case of the flu was a virtual death sentence. We're back to the days when 4/5 children died before their fifth birthday, and giving birth was highly likely to kill the mother, and often the baby as well.

Stress - emotional and physical - has a tremendous impact on the immune system, and the survivors are living under constant, intense stress, both emotionally and physically. The emotional stress is obvious - constantly fighting to survive, watching loved ones die left and right, and unrelenting uncertainty and fear - but the physical stress is probably more detrimental. Limited food resources, drastically reduced variety in the diet, limited access to drinking water, very little medicine - all of these things (which were slightly improved when the group reached Alexandria, but which are still a serious problem, even there) are taking their toll and making our protagonists less resistant to disease, and more susceptible to complications from injuries that we, in the pre-apocalyptic world, consider trivial.

Furthermore, the TWD Wikia points out:

The [zombie] pathogen itself does not kill its hosts, but it seems to weaken their immune systems considerably, to the point where even minor illnesses are far more fatal than normal to humans.

This is plausible, in light of what we've seen thus far on the show and in the comics, but had yet to be confirmed or explained.

In short, it is much more difficult to stay alive, even if we ignore the threat posed by zombies and hostile humans. The things that might make us, in the real world, mildly uncomfortable, are likely to become life-threatening situations for the survivors on the show.

The Kill Count Prior to Season Five:

There are statistics for onscreen zombie and human deaths on the show up to the end of season four. This site lists the kill count in seasons 1 through 4; I don't know if their numbers are exactly right, but assuming that they aren't wildly inaccurate, they are good enough for our purposes. I have taken the liberty of breaking down the data and creating a spreadsheet.

enter image description here

The members of the group who are listed by name in the chart have killed 956 zombies and 39 people. Other human characters have killed 229 zombies and 136 people. Zombies have killed 78 people.

The grand total:

1,185 zombies killed
253 people killed

In the comic books, and to a lesser extent on the show, Rick's group are depicted as being among the best fighters left in the world. They have repeatedly fought off much larger forces of zombies and bad guys alike. In comparison, people who aren't in Rick's group are usually zombie fodder.

The first four seasons of The Walking Dead represent perhaps 2 years of time. In 2 years, the best zombie killers around have only managed to kill 956 zombies. This does not bode well for humanity.

How Many People Versus How Many Zombies?

The blurb for the tenth issue of the comic book says:

After the ordeal Rick has endured last issue, he sets out to find safer shelter. More is learned about the zombies that now out-number us 5000 to 1, but when it comes to some things, it's better not to know. Rick begins to wonder if there is a light at the end of the tunnel his life has become. Even if there is, how can he ever expect to make it there?

In Issue #10, Carl is brought to Herschel's farm for the first time, having just been shot by Otis. This corresponds to the second episode of Season Two of the show. The second episode of Season Two takes place about a week after Rick wakes up in the hospital, and a few weeks since the outbreak began.

This means that within weeks of the beginning of the outbreak, only 0.02% of the population was still alive. How does this affect the data?

0.02% Humans, 99.98% Zombies:

  • Worldwide: 1,400,000 people against 6,998,600,000 zombies.

  • United States: 60,000 people against 299,940,000 zombies.


We're doomed, and trying to survive is just playing for time.

  • Excellent answer, but is it answering the right question? I took the question to mean "How are so many people becoming walkers if criteria X Y and Z all need filled and it's unlikely that all three will happen" – Robert Wertz Dec 1 '15 at 18:52
  • Zombies appear to lose potency, and they are also incredibly easy to kill when the humans are prepared. All that's really needed is a few hundred humans to go on major zombie killing sprees while wearing ski gear. That said, I'm pretty sure there's someone out in a remote part of Alaska or Canada who doesn't even know anything changed. – user40790 Dec 1 '15 at 18:57
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    Not that it would make much statistical difference, but surely the characters that join later have killed many more than what is seen. Abraham and Michonne in particular I am sure have much higher zombie kill counts. Abraham also killed at least 4 people, but I do not remember when we are told that. – Dave Johnson Dec 1 '15 at 19:06
  • @RobertWertz - patience, young padawan. I was still working on the answer. – Wad Cheber Dec 1 '15 at 19:09
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    @WadCheber: You missed another instance of 0.2%, looks good otherwise. :) – Junuxx Dec 1 '15 at 20:49

I am unaware of a canon answer, since in the show at least, we haven't heard from anyone who was working to contain the plague on a national level(the CDC guy was trying to cure the disease, he wasn't really making policy decisions). However, I think we can make some strong guesses:

Option 1: I think you under-estimate how many people would have become walkers by "surviving" being bitten. In Alexandria, many people end up cut off and surrounded by one of the larger hordes in the series. Even so, a fair number of them escape the horde after being bitten. Unless you actually fall into a crowd or get cornered, its not too hard to force your way through a crowd of zombies. And this is against a full herd, early in the infection there would have been fewer large groups of zombies so presumably it would have been even easier to escape after being bitten.

Option 2: In "Fear the Walking Dead" we see how severe riots and civil unrest became in cities early in the outbreak. Not only was there crime and looting, but the military was shown to be willing to use draconian tactics to maintain some semblance of control. Add to this the re-emergence of previously controllable disease, people who require 1st world medications to survive, lethal injuries/infection, contaminated water, starvation, exposure etc, and you have a fairly massive body count.

I think these two general factors account for the majority of walkers. When you consider that the longest known survival time after being bitten is two days(http://walkingdead.wikia.com/wiki/Jim_(TV_Series)) it is easy to imagine the infected population exploding very quickly, especially as people who were bitten fled with the un-infected(flight 462) or hid bites from fellow survivors(Jim, among others).


TS-19, The Walking Dead Wiki:

As Jenner takes a blood sample from Andrea, she asks him what the point is. "If we were infected, we'd all be running a fever,"

Beside the Dying Fire, The Walking Dead Wiki:

Daryl brings up Randall, explaining that he turned without being bitten. "We're all infected," Rick says solemnly. "At the CDC, Jenner told me. Whatever it is, we all carry it."

From that explanation, I assumed that most people got the virus and died from it, thus reanimating as zombies. The few survivors have the virus in their blood but it's dormant and only reactivates when bitten, thus receiving a large viral load from the bite.

That theory is collaborated by a couple of things:

  1. When Hershel was bitten, Rick amputated his leg to stop the spread of the infection
  2. You can get infected by a simple scratch
  • 1
    The bite doesn't transmit the zombie virus: 'The rule is WHATEVER it is that causes the zombies, is something everyone already has. If you stub your toe, get an infection and die, you turn into a zombie, UNLESS your brain is damaged. If someone shoots you in the head and you die, you're dead. A zombie bite kills you because of infection, or blood loss, not because of the zombie "virus."' - Robert Kirkman – Wad Cheber Dec 1 '15 at 22:13
  • No one is immune, and I don't think it has been explicitly stated that the zombie virus is airborne, in either the comics or the show. – Wad Cheber Dec 1 '15 at 22:14
  • "Just to get this on record once and for all... and it is complicated, I know... here's how zombification works. Whatever makes people come back as zombies after they die--it's inside them. It's inside everyone. No matter how anybody dies, as long as the brain is intact... they turn into a zombie. Well... bites, and direct to blood contact with zombie gunk, [...] causes death. It's a strong infection that leads to fever that kills someone. Then the "virus" or whatever is already in them... turns them into a zombie." —Robert Kirkman – Wad Cheber Dec 1 '15 at 22:15
  • The infections caused by bites are mundane, real-world pathogens, like sepsis. The zombie virus doesn't kill anyone, it just brings people back after they die from other causes. – Wad Cheber Dec 1 '15 at 22:16
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    Can't right now. Another thought to consider - when Hershel was bitten, Rick amputated his leg. Obviously that was done to stop the virus, the amputation was significantly more serious wound than a mere bite. Not to mention that people can get infected simply by being scratched. – ventsyv Dec 1 '15 at 22:42

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