What Are The Chances Of There Being A Zombie Apocalypse In Your Lifetime?

31 Jul

We all know that there are zombie apocalypse preppers out there, and many think that they’re simply wasting their time because a zombie apocalypse could never happen. However, what if they are right and it is the rest of us that are wrong? Isn’t it better to be prepared and have nothing happen, than not be prepared and get caught out?

Well, that all depends on the risk of it happening. As with anything in life, we need to know how likely (or unlikely) something is to happen before we can decide whether we should take precautions and how much time and money we should put into them. Yet, how is this risk calculated? How on earth would you calculate the risk of being killed in a zombie apocalypse?

Let’s take a look at how risk is calculated by looking at mortality risk. This is effectively the chance of something killing a person multiplied by the number of times it can potentially happen over a given length of time. This is worked out by taking the number of people that die from a specific cause, dividing it by the total number of people exposed to give the chance of each individual dying because of it. This figure is then divided by a measure of time to get the death rate. This death rate can then be expressed in a number of ways, but the most common is the micromort. What is a micromort? It’s a one in a million chance of dying from a specific cause (which is about the same chance of throwing 20 coins into the air and them all landing heads up). So that’s the theory, but how does it work in practice?

Well, if we wanted to work out the risk of being murdered in the UK (where I live), we can work this out as follows: There were 640 murders in the UK in 2011. This is out of a population around 61.37 million. So we divide 640 by 61.37 million and find that each individual has a 0.00104% chance of being murdered each year. To work out this value in micromorts, we then multiply it by a million and find that the risk dying from an external cause in the UK each year is 10.4 micromorts.

But is 10.4 micromorts a lot or a little? To find that out, we need to compare that to other causes of death. The chances of dying because you spend one night in hospital in England is 75 micromorts. So, each year, you’re 7.5 times more likely to die from having to spend a night in hospital in England than of being murdered. Similarly, you’ve got about the same chance of dying each time you’re were given a general anesthetic as being murdered (10 vs 10.4 micromorts) and you’re eight times more likely to die each time you give birth in Britain than being murdered in any one year (and you’re 17 times more likely to die while giving birth in the US than of being murdered in any one year in the UK). Of course, that’s just the UK. In The US, the risk of being murdered is almost five times higher than in the UK at 48 micromorts, while in Canada it’s 16 micromorts.

So, that’s how risk calculations and micromorts work, but how do we apply this to work out the chances of a zombie apocalypse happening and, indeed, the chances of you dying in a zombie apocalypse? First, we need to set the parameters for our zombie apocalypse. When we do this, we need to be realistic about this and follow the rules of how the world actually works. This means it can’t be dead people coming back to life, that’s just biologically impossible. However, we can have diseases which take over people’s brains and make them act like flesh-eating zombies from the movies that will attack any other human they get hold of. There’s a surprisingly large number of real diseases that can do that, but I’m going to focus on one real ‘zombie’ disease and that’s rabies.

Rabies really does take over people’s brains and make them act violently to others, and it’s transmitted by bites, but there’s two things which stop it creating a zombie apocalypse. The first is that it’s 100% fatal so anyone infected with it dies (at least if they’re not treated before they start showing symptoms). The second is that it’s not easily transmitted from one person to another. In fact, there are no known cases of human-to-human transmission. This isn’t because the disease isn’t infectious, but rather because human teeth are actually remarkably poor at being able to bite through human skin. Also, for obvious reasons, we tend to keep well clear of people who are staggering around, foaming at the mouth and trying to bite us. Together, these make it hard for humans to transmit the disease to each other.

But what if this were to change? Diseases mutate all the time and it is not inconceivable that the rabies virus could mutate in such a way that would make it less fatal, and easier to transmit. Indeed, there is already evidence that some strains of rabies are already evolving towards being able to spread much faster and more efficiently between animals without the need for bites.

So what’s the risk of this happening? If we assume that the natural mutation rate of the rabies virus is 0.00001, that is for every 10,000 times it copies itself, there’s one mutation which changes something about how it operates. Some of these mutations may be good for the virus and some may be bad, but we’re looking for very specific mutations that would change the virus in a very specific way. Thus, out of all the mutations, maybe only one in a billion might have the effect we’re interested in. Taken together, that means that there’d be a 1 in a ten trillion chance of just the right mutation happening. You’d think that would make it so unlikely that we’d never need to worry about it, but you’d be wrong. Why? Because of the law of very large numbers.

The law of very large numbers means that even very unlikely outcomes can happen if we do something often enough. You see, there isn’t just one rabies viral particle in anyone who’s infected, but millions of them, all reproducing, all the time and so all at risk of mutating in just the right way. If we take the figure of 1 million as a highly conservative estimate of the number of viral particles in any one person with rabies, and say that each one reproduces once every 10 days (again highly conservative), then within any infected individual there’s a 0.0001% chance of the required mutations occurring. Still a very small chance, but much more likely than 1 in a 10 trillion, and that’s just in each 10 day reproductive cycle in each person.

A typical person may survive for 12 weeks after infection, so there would be around eight such reproductive cycles in each person. This brings the risk of the required mutation happening in any one infected person up to 0.0008%, and around 50,000 people are infected each year, which means that when all the viruses across all the people are considered, there’s a 42% chance of the mutations happening in any one year. Just because we’re dealing with very large numbers, we’ve suddenly gone from a risk that’s infinitesimally small to one that’s almost the same as guessing whether a coin will land heads up or tails.

Rather shockingly, that means there should be a rabies-related zombie apocalypse once every 2.4 years. If we assume that 80% of the world’s population of seven billion would be killed or infected if a zombie apocalypse were to happen, then this would equate to 2.3 billion micromorts each and every year. Now, compared to out 10.4 micromorts for being murdered in the UK, that’s pretty damn high. It means that in any one year, you’d be almost 50 million times more likely to be killed in a zombie apocalypse than of being murdered, which is pretty wild and suggests that most people are worrying about all the wrong things when it comes to what’s likely to kill them.

Now, at this point, you might be thinking to yourself that you don’t remember any zombie apocalypses happening recently, and even though you’ve been pretty busy for the last 2.4 years, you’re pretty sure you’d remember 80% of the world’s population getting wiped out. So, if these are the odds, why hasn’t it happened?

Well, this is most likely to do with two things. The mutations which might make rabies more transmissible between humans might also make the virus itself less fit, and that means any viruses which carry the mutations are pushed out by those which don’t so they cannot get enough of a toehold to become the dominant type of virus in any one individual. This in itself would be enough to stop any copies of the virus which carry the required mutations from breaking free and rampaging across the planet. In other words, the virus itself might be keeping dangerous mutations in check simply because they have to compete with other versions without the same mutations for the limited resource which is the human body.

Secondly, it might be that the rabies virus simply cannot generate the mutations needed to turn it into a supervirus capable of taking over the planet. A mutation can only change what a viral gene already does. It cannot suddenly create a brand new gene that does something completely different (like genetic engineering can do). It’s just like a car, you can fiddle with the engine to make it go faster, but you can’t, just by tweaking it here and there, suddenly turn it into an aeroplane. There just aren’t the existing building block there to do that. So it might be with the rabies virus, that no amount of tweaking could ever turn it into a civilisation-destroying, zombie-apocalypse-creating megavirus that would wipe humanity from the face of the planet, but we don’t know for sure. Maybe it’s not a one in 10 trillion chance mutation, but one in a ten thousand trillion. Very unlikely, but still possible given enough time (in this case about 238,000 years – short by our lifespans, but not long in terms of how long humans have been roaming the planet, picking up rabies from other species – and that still gives it a value of 2384 micromorts per year, still high in comparison to the risk of being murdered).

So what can we learn from all this? Well, firstly, if you do anything often enough, even the most unlikely events can happen, and what has a very low probability of happening to each individual, might be quite likely to happen when there are 7 billion people on the planet – after all, someone has to guess the right numbers and win the lottery, it’s just very unlikely to be you.

Secondly, just because you can calculate the likelihood of something happening, it doesn’t necessarily mean that it will. In particular, if you don’t take everything into account properly, your calculations can go very wrong (in case of the above, assuming that the types of mutations required to turn rabies into a zombie-apocalypse creating megavirus are actually possible).

Finally, as long as you’re not talking about risen-from-the-dead type zombies, a zombie apocalypse is possible. There are diseases out there which can take over the brain and turn humans into zombie-like killers and it’s biologically feasible that they could mutate to create a highly infectious zombie-like disease. Given the numbers, even if such a mutation was very, very unlikely, there’s still a possibility that it will happen eventually. This leads on to a very interesting question: What’s stopping this happening?

In other words, if the law of very large numbers makes it highly likely that diseases will spontaneously generate the mutations needed to spark a global epidemic, why are they so rare? If we can work that out, then we could hopefully move towards stopping those that do occur. Here, I’m not talking about a zombie apocalypse, but real world problems, like HIV, flu, ebola, SARS and MERS, and others which we’ve still yet to discover, and which have the power to destroy large proportions of humanity if and when they go pandemic. What makes these diseases suddenly appear as if out of nowhere and start running riot through human communities and populations, while others, like rabies, simply remain bubbling away slowly in the background? This isn’t just idle speculation, but something which is fundamental to our understanding of diseases, and indeed our future on this planet.

Who would have thought that’s where we’d have ended up when we started trying to work out if zombie apocalypse preppers were wasting their time or not?


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From the author of For Those In Peril On The Sea, a tale of post-apocalyptic survival in a world where zombie-like infected rule the land and all the last few human survivors can do is stay on their boats and try to survive. Now available in print and as a Kindle ebook. Click here or visit www.forthoseinperil.net to find out more. To download a preview of the first three chapters, click here.

To read the Foreword Clarion Review of For Those In Peril On The Sea (where it scored five stars out of five) click here.

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