This isn’t exactly meant to be a how-to guide for bringing about the end of mankind, but in many ways it reads like one. I’ve outlined the (as I see it) most likely and zombie-like neurology that could form through various diseases, and how these diseases may emerge en masse. While the exact definition of a zombie is both fictional and hotly debated (a weird but predicable truth), the following cocktail of afflictions gets us pretty close. What we arrive at is not exactly Night of the Living Dead, but a more plausible version of what to expect if (or when?) the time comes. Good luck.

Unsurprisingly, there isn’t (yet) a disease that embodies the entire zombie symptomatology, so we’ll have to throw a few things together. We will start with Klüver-Bucy syndrome (KBS) and make additions from there on.

KBS typically results in an individual exhibiting at least 3 of the 5 following symptoms(1) –

1. Placidity.

2. Hyperphagia (compulsive eating, often of inappropriate foodstuffs – geddit?).

3. Hyperorality (obsessive oral examination of objects).

4. Hypersexuality (you can guess).

5. Visual agnosia (inability to recognise familiar objects or people).

As an additional note, three other symptoms are somewhat inconsistently found– hypermetamorphosis(2,3) (inability to inhibit attention from new stimuli), diminished emotional effect(4) and memory loss(5).

If we take the symptoms most closely linked to zombies – hyperphagia, hyperorality, visual agnosia and diminished emotional effect, we have an individual who can’t recognise people, places or the emotional content inherent within them and has an extreme tendency to put things in their mouth and often eat them. It’s a good start, but there’s obviously a long way to go. (I should also add that in no way am I suggesting that people who suffer from KBS are closer to zombies than the average person, just that a certain arrangement of symptoms present interesting similarities – a person with KBS is still very much a wonderful human being – if a little bitey).

On an even further note, I’d like to add that while this exact arrangement of the syndrome (and subsequent afflictions that give us a zombie caricature) are extremely unlikely, I’m working on the premise that some of the individuals will become zombie-like enough that they will just start eating / killing / infecting which would remove the non-identical sufferers from the population (and us, too).

The most likely cause of KBS (for the purposes of creating a zombie apocalypse, at least) is meningoencephalitis(6). This is a form of meningitis that causes massive inflammation of the brain and can be triggered by a buffet of different bacterial strains(7) and (at least) 7 different viruses(8). The bacteria Neisseria meningitides has the advantage of being spread through respiratory actions(9), while six of the thirteen serogroups(10) (basically different forms of the same bacteria) are capable of causing an epidemic. A hypervirulent strain was found in China as recently as 2006(11). If an epidemic was triggered then we can assume that a small but significant portion of the infected will develop meningoencephalitis, leading to KBS with the symptomatology that we’re looking for. It seems unlikely we’ll have hordes of endless, mindless, flesh-desiring guys and gals, but maybe a few. It’s a start, but obviously more is needed.

For the next step, we need something that will tip the balance a bit more towards fully-fledged Zombiehood. This is where things get a bit more difficult logistics wise (obviously it was so simple and easy before…). Lead poisoning seems like our best (worst?) bet. Acute lead poisoning usually and conveniently leads to the same symptoms of chronic lead poisoning(12). This does a variety of nasty things, but most notably will diminish brain size, particularly within the anterior cingulate cortex (ACC)(13,14). This area is involved in detecting errors and correcting them, as well as reinforcing behaviours that give rise to rewards(15) – perhaps most importantly though is the involvement with acute pain(16) and emotional awareness(17). Knock this out (or diminish it, as we will) and we’ve got someone who’s not great at detecting errors, doesn’t feel much pain, and has some trouble with reading the emotion in a scene. Lead poisoning also often causes hypertension(18) which, in turn, increases aggressive behaviour(19) and the transmission of opoids(20), thereby decreasing the pain response even further. Add this to the lead-coloured skin that starts to emerge and we’re definitely getting there.

To top this off I’m going for the obvious – the rabies virus(21). Similar to meningoencephalitis, rabies causes inflammation of the brain (most of the time), and has relatively predictable effects. The incubation period (the time between contracting the virus and exhibiting symptoms of the disease) lasts around a few weeks and without treatment following this, the patient will rarely survive another week or two. The symptoms list can be pretty varied, but let’s see what it usually gives us:

Fluctuating periods of extreme agitation and depression (the agitation is crucial, but obviously the depression less so).

Aerophobia (anxious reactions to, well, air – gusts of winds particularly).

Hydrophobia (anxious responses to water and an apparent inability to drink. Interestingly, thirstiness is found to increase aggression (22) ).

). Spasms, fearful facial expressions and ’hypersalivation’ (well, this would definitely add to the appearance).

Untreated, this will give way to a coma and death will follow shortly after. But what does this all mean? (Apart from wet and windy coastal towns being the perfect refuge in light of a zombie attack). Our ‘realistic’ zombie will ideally (ideally doesn’t really seem like the right word, but work with me) show the following:

Be emotionally numb.

Have a tendency to bite and eat things.

Be unable to correct their errors or improve their actions (not improving their actions is a fairly big deal).

Not feel pain.

Be aggressive.

Be scared of the wind and water.

Have grey skin, twitch, drool and look pretty scared.

Be able to spread the disease somewhat (obviously it can’t spread lead poisoning, but the other two would probably be contracted through a cheeky bite or two).

Added bonus – leprosy(23). While only 5% of the population aren’t immune to this disease (pretty cool, right?), the skin lesions that form certainly add an extra layer of credibility to the appearance (with apologies to people with leprosy), and the increased chance of infections are likely to lead to some gangrenous limbs coming unstuck here and there.

Obviously this isn’t a perfect match to how we view a zombie – here are just a few examples of things we’re missing:

Zombie-like sounds – these guys can still talk! (Although probably less so amidst the pain of encephalisation and rabies…).

A particular lust for human flesh – sure they’re hungry but their voracious appetite isn’t aimed at tasty humans, just a general predisposition to give things a bite here and there.

Undead idiocy – despite the brain damage, it’s unlikely our afflicted adversaries would forget how to open a door or simply talk to someone, which leads me to a convenient extrapolation (read – measly justification) as to how this would all pan out, and why none of it would end well.

DAY 1: So, here we are in town X (or ‘Z’, if you feel like it). A huge amount of lead has leaked into the water supply for a couple of weeks now (or maybe years – and don’t ask me how, you fucking terrorist). And then there was that outbreak of Neisseria meningitides that’s suddenly spreading round rapidly - and to top things off - a rabid pack of dogs has emerged and they’re just biting a metric fuck-ton of people. It’s the beginning.

DAY 7: There’s quite a few people infected with all three things now – the rabies hasn’t kicked in yet, but a lot of people are dying from the meningoencephalitis, or starting to show some weird symptoms. People are getting angry generally and even more so with the headaches and brain-shrinkage from the lead. The bites begin. A couple, frolicking in a golden field perhaps (hey, why not?) are getting close, when one of them finds themselves staring earnestly at the other’s neck – a sudden thought comes to them – I wonder what that would feel like in my mouth? I wonder what it tastes like? So they give it a chomp and immediately regret it when the other is pretty pissed off about it. This seemingly insignificant act becomes viral news after the farmer films this happening – particularly the bit where the biter is stumbling along the field, grey-skinned, drooling blood, twitching and looking pissed off.

DAY 14: Now things are getting serious with the rabies – people are angry. They’re also foaming at the mouth and not looking too great in other ways. The bites are increasing in their seemingly innocent fashion. There’s the beginning of an exodus as people start to believe the zombie rumours, or at least, they know people are ill and a few take the initiative, why not have a short-break? This is also the beginning of the spread of the disease (minus the dastardly lead poison), alongside some rabid feral dogs that make a runner.

DAY 21: A few people are already dying from the untreated rabies, but for most it’s still taking it’s time – people aren’t exactly eating well though and some are developing hypothyroidism(24) – their skin is worsening and their nails are thinning – most importantly though, their voice is now slow and croaky. These guys are slowing down as less epinephrine is released into the blood. People are scared of them and the hospitals are overrun. Their thirst worsens but they don’t want to be near water. Provocations towards the afflicted grow and fights break out – the biting increases and the infections too. Everyone in town X is sick in some way and the spread of rabid biting is increasing across the country, and soon across other countries.

DAY 28: The first wave is mostly dead now; the mixture of all three diseases kills almost everybody infected eventually. Meanwhile, the lesser form of the disease has spread and the panic is reaching a peak. People are convinced of the undead as video after video shows snarling, bloody, grey-skinned men chewing in desperation on lifeless (or lively) bodies. It isn’t the whole story - other footage shows infected people eating normally, others asking for help – but all this is ignored as the zombie plot sells and their voices (quite literally) diminish in confusion and disease.

DAY 35: Too scared to rationalise, people are locking themselves down, away from anyone who seems infected – we’ve all seen what happens so why take the risk? Now free of lead-poisoning, the infected are no longer grey-skinned and numb to pain – they’re even less bad at reading the emotion in a scene. This makes it all a whole lot worse as your neighbour freaks out when some foam appears in your mouth and takes a liking to a trigger on a shotgun. Paranoia now rules above reason.

DAY 42: Quarantine and lock-downs go into effect and entire cities go missing. No-one wants to take the chance anymore. The disease spread predictably – huge mortality rates with apparent zombie-like ‘survivors’ who themselves die less than a month later. People are eager to capitalise – looting and riots become commonplace as the panic and confusion remains. The cycle continues and the population diminishes.

You can pretty much see how this all ends.

References

1. http://onlinelibrary.wiley.com/doi/10.1002/cne.901030202/abstract

2. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3272911/?tool=pmcentrez

3. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1064170/

4. http://archneurpsyc.jamanetwork.com/article.aspx?articleid=648265

5. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1032530/

6. http://www.ncbi.nlm.nih.gov/pubmed/168031

7. http://www.ncbi.nlm.nih.gov/pubmed/21180625

8. http://www.ncbi.nlm.nih.gov/pubmed/20236175

9. http://www.who.int/mediacentre/factsheets/fs141/en/

10. http://wwwnc.cdc.gov/eid/article/17/3/10-1329_article.htm

11. http://www.ncbi.nlm.nih.gov/pubmed/16458767

12. http://books.google.se/books/about/Goodman_and_Gilman_s_the_pharmacological.html?id=SGQQAQAAMAAJ&redir_esc=y

13. http://www.plosmedicine.org/article/info%3Adoi%2F10.1371%2Fjournal.pmed.0050112

14. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2538609/?tool=pmcentrez

15. http://www.sciencemag.org/content/280/5364/747.short

16. http://www.molecularpain.com/content/6/1/42

17. http://www.cell.com/trends/cognitive-sciences/abstract/S1364-6613(10)00252-4

18. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1474616/?tool=pmcentrez

19. http://www.clinchem.org/content/44/2/366.full

20. http://www.sciencedirect.com/science/article/pii/0006899380910550

21. http://www.sciencedirect.com.ezproxy.its.uu.se/science/article/pii/S1474442202000418

22. http://www.sciencedirect.com/science/article/pii/S0091677373800847

23. http://books.google.se/books/about/Handbook_of_leprosy.html?id=NeJrAAAAMAAJ&redir_esc=y

24. http://endometabol.com/?page=article&article_id=2092