Zombies are everywhere.
Now, before you reach for the chainsaw, I of course mean in popular culture. Shuffling reanimated corpses with the taste for brains can now be found in cinema, TV, books, comics, video games, art, music and beyond. After Bela Lugosi had appeared in the 1932 film "White Zombie", if you had told him that in 2012 you could give your young child zombie candy to put in their zombie lunchbox while they played with zombie toys, he would have probably assumed that you were insane. He would have probably been more inclined to believe you if you had simply said that in 2012 we live on the moon.
But could zombies be a genuine threat one day?
Is it time to stock up on shotgun shells, red and green herbs and good, strong cricket bats in preparation for the time "when there is no more room in Hell," and when "the dead shall walk the Earth"?
Er, well, no, not at all.
The thought of something as complex as the human body being reanimated from a prolonged period of cellular death and putrefaction is (though extremely attractive to the entertainment industry) as close to "impossible" as my science brain will allow me to think.
So why the cultural fascination?
So, why do I, as a scientist, and thus so keenly aware of the pseudo-science nonsense that begins nearly every zombie story, still proudly house my mobile phone within a "Keep Calm and Kill Zombies" case?
In fact I would treat with extreme suspicion any scientist around my age that hasn't had a serious conversation with their colleagues about the best strategies during a zombie apocalypse.
Thankfully I've yet to meet such a scientist. Personally, I believe that the reason zombies, along with the other members of the "Big Three" of modern monsters (vampires and werewolves/beast-men), still command such collective curiosity can be summed up in one word: control.
In my opinion people love stories involving these contemporary demons as they symbolise the consequences of losing control of oneself. Vampires are often used as an allegory of losing self-control in terms of sexuality and lust.
Werewolves show us what it would be like to give in to aggression. And finally, zombies and a zombie apocalypse teach us about losing our grip on our individuality (as one of the undead horde), and on society at large (as a "survivor"). It's equally fun and frightening to think about what would happen if we were to lose the main thing that appears to set us apart from the rest of nature - to flirt with losing our free will.
Interestingly (read "somewhat terrifyingly"), it is in this core aspect where there is more of a meeting of both fantasy and science.
In the natural world there are a staggering number of examples where infections are able to hijack the mind of their host organism, and get them to do their bidding in attempts to spread the disease to another unlucky individual.
Now for the science bit
Examples of this natural phenomenon are found in the parasitic wasps of the genus Glyptapanteles. This parasite also has an analogue in science fiction. I would say most people that would read this article are aware of the "Alien" movie franchise, and have a rough idea of the life-cycle of the film's titular "Alien", the Xenomorph. Human beings are impregnated with a Xenomorph embryo that eventually, er, "emerges" (rather messily) from its human host.
This "chest-burster" skulks around in the shadows until it grows to become a fully-fledged reason that the USS Nostromo should have stayed away from LV-426.
Glyptapanteles wasps also act like this, though using caterpillars (thankfully not humans) as a host. And frankly they actually do it a whole lot better than any Xenomorph. Any self-respecting colonial marine knows that one of the best times for a "bug-hunt" is once the chest-burster has emerged. It is very vulnerable in that new-born state and is forced to hide before it develops into the cosmic killing machine that has been bothering Ripley and friends since 1979. Glyptapanteles wasps overcome this weakness by completely changing the host caterpillar's behaviour.
Once the wasp embryos emerge (not as messily) the caterpillar stands guard over them as their own personal zombie bodyguard, flailing wildly if anything approaches to "shoo" away potential predators. The caterpillar will even cover the wasp embryos in its own silk to protect them while they pupate. By the time the new wasps appear from their cocoons the caterpillar simply drops dead, starved and exhausted.
Many other species of parasite must infect two very different hosts to complete a full life-cycle, and have developed ingenious ways of making this jump into a different host. For example, the Lancet liver fluke (Dicrocoelium dendriticum) infects snails, and then ants who feed on snail slime. These ants are then inadvertently consumed by grazing mammals such as sheep or cows, which become the third host for the parasite. The fluke is then expelled in the mammal's faeces, which is then fed upon by snails, and the cycle continues. However, there is a slight problem. As the ants are generally either in their nests or crawling very low along the ground, consequently the chance of them being eaten by a passing herbivore is pretty slim.
So the Lancet fluke steps in to tip the odds in its favour.
As the sun begins to go down an ant infected with this parasite will abandon its queen and colony and climb to the top of a blade of grass, clamp its jaws down and hang there; waiting to be picked up by an unsuspecting (and hungry) ruminant in its next mouthful of grass. What is even odder about this situation is the fluke is able to turn its control on and off as it sees fit. If the ant is exposed to prolonged, direct sunshine the insect will dry out and die, meaning the death of the fluke too. As a result, the parasite releases its control as the sun begins to rise. Like a guy once bitten by a large wolf outside the Slaughtered Lamb pub, the unfortunate ant wakes up naked in the middle of a field, wondering how it got there, before going back to its normal day-to-day "ant-ing". Unlike the protagonist of "American Werewolf in London" however, this happens every dusk and dawn for even as long as a year, not just at a full moon. And the ant doesn't wake up covered in the blood of its innocent human victims, obviously.
So far, so, well, creepy. However, these two examples have shown behavioural control of insects rather than of mammals. Additionally neither of the examples showed infections from a virus, a common theme in zombie lore. Biologically, a virus is so simple, we don't even class it as alive, strictly speaking.
So there's no way something as "basic" as a virus could affect the behaviour of something so complex as a mammal, let alone a human being? Right?
All the rage
As Stephen King's "Cujo" taught us, the rabies virus is adept at altering the behaviour of mammals, including humans. Rabies stimulates overproduction of saliva (where the viral particles reside) and then promotes violent, aggressive and, well, "bitey" behaviour resulting in a greater chance for the virus to be passed on to another host. Thankfully a vaccine is available and many control measures are in place so that this disease is not particularly widespread.
My final example is a parasite named Toxoplasma gondii, cause of the disease toxoplasmosis.
More advanced than a virus or bacteria, but still microscopic, this parasite infects mammals, though its main host is the cat. Unlike rabies, it is very widespread, with some areas of the globe having an estimated 75% of the human population infected with the parasite!
In infected rodents it has an amazing ability to very specifically alter their behaviour.
The fear of predatory cats by rodents is actually known as a "fixed-action pattern", or in other words is hard-wired to such an extent that even lab mice that haven't encountered a cat for over 100 generations will still avoid the smell of felines. Infection with T. gondii turns that "off" so that they are unafraid of (and in some cases, actually attracted to) cat odour; while otherwise the rodent is completely normal, and near indistinguishable from its uninfected peers.
This greatly increases the possibility of the rodent being eaten by a cat, and thus the infection being passed over to the predator. While the exact mechanism is unknown, it appears that the parasite accumulates in the amygdala, part of the brain associated with memory and emotional reaction. There, it appears to raise the amount of the neurotransmitter dopamine within the mammal, important in reward-driven learning.
Other than generating extremely short episodes of "Tom and Jerry", this infection has greater implications to us as the parasite can also be passed to humans, through eating uncooked meat or by contact with cat faeces. Normally this disease results in mild flu-like symptoms, though can be fatal to those with a compromised immune system. Fascinatingly, this infection can also alter human behaviour. There is evidence that it makes us less cautious and more likely to take risks. Beyond that, toxoplasmosis has also shown links to schizophrenia, as well as anxiety, depression, self-harm and even suicide. In fact, Professor Jaroslav Flegr from the Czech Republic has even gone as far as to suggest that the disease may be responsible for up to a million car crashes a year. Our minds aren't exactly the fortresses we hoped they were.
So we come back to an earlier question, though now we ask not "could zombies become a reality", but "could there be infections out there which would change our behaviour, with little room for self-control"? Now the answer is a resounding "yes", as these already exist! We are, of course, very far away from the zombie apocalypse - however varied and bizarre the effects of these diseases are on humans, "cannibalism" is thankfully not one of them.
Then again, if there wasn't at least a slight grain of scientific truth involved, science fiction would just be, well, fiction...
Andrew Voak is a research scientist finishing his PhD at Queen Mary, University of London, who studies a parasite that causes the human (non-zombie) disease leishmaniasis. He advises you to aim for the brain.