Zombies have dominated science fiction for years. But they don't actually exist, right? Wrong. There are several real-life diseases that could make you act like a zombie.


If we're going to talk about zombie-like diseases, we first need to decide what the symptoms of being a zombie actually are. Obviously, the big one - you know, being literally, actually (un)dead - isn't something with any real world medical parallels, so we'll just have to restrict ourselves to diseases that make people act like the walking dead. That would include traits like rotting or dead flesh, a trance-like state that would rob people of any sign of higher cognitive function, an inability to communicate in anything more than moans and grunts, a slow, shuffling gait, and (if we're really lucky) a taste for human brains, or at the very least the desire to bite people.

Is there a single disease that can do all that? Well...no. But there are a whole heap of diseases that can do quite a few of those, and that's plenty terrifying enough. Indeed, let's start with the most horrific possibility of them all:

Sleeping sickness

Sleeping sickness is the stuff nightmares are made of. The headline of this BBC News article from 2005 pretty much says it all: "The disease that makes people zombies." Prevalent in Africa, sleeping sickness is caused by the parasite Trypanosoma brucei and transmitted by the tsetse fly.


Interviewed for the BBC News piece, Professor Sanjeev Krishna of the University of London and a doctor at a hospital in Lucala, Angola, explained the horrible progression of the disease:

"At first it will cause headaches, aching muscles and maybe itching. But in the late stages, when the parasites have invaded the brain, the signs become more obvious and ominous. Victims find it hard to concentrate. They become irritable, their speech is slurred and they stop eating. Their daily rhythm becomes disrupted to such an extent that they can't sleep at night and find it almost impossible to stay awake during the day. It even becomes very hard for them to do simple mental tasks, such as drawing a straight line. This is an infection that carries nightmarish qualities, reducing many of its victims to a zombie-like state before they go into a coma and die. Those that do survive can be left with irreparable brain damage."

Worse, there are still no vaccines or ways to prevent infection occurring once the tsetse fly bites a person. Even the available treatments are - to be charitable - less than perfect. Melarsoprol is one of the few treatments available (and that rather dubiously assumes that the average infected person has access to any medical care), but it's over fifty years old and contains enough arsenic to kill 1 in 20 people that are treated with it. And even if a patient survives the ordeal, they remain at risk of contracting the disease again later.

About 50,000 to 70,000 people die of sleeping sickness every year, although Krishna suspected that estimate was actually much too low. In Uganda, one in every three people is at risk of getting the disease, and some sixty million people remain under constant threat. So then, there are about 50,000 examples of the walking dead each year, although (perhaps mercifully) they don't remain walking for very long.


Thankfully, there is some hope. The genome of Trypanosoma brucei was sequenced this April, and it is currently being compared to another strain of the parasite that only affects cows. Researchers at the Sanger Institute who carried out the sequencing hope this comparison will yield hugely useful data on just why one strain infects humans and the other doesn't. This could pave the way for new drugs that would greatly reduce the mortality of sleeping sickness and hopefully decrease the incidence of the most severe, zombie-like symptoms in the afflicted.

Rabies

There isn't a disease, be it mental or physiological, that makes people want to eat other people, at least none as currently recognized by medical science. (Cannibalism isn't considered a mental illness in its own right, but rather as a part of a larger web of psychoses.) There are certain culture-specific mental conditions - Wendigo psychosis, observed in certain native American peoples, is one of the better examples - that make people think they are turning into cannibals, but that's about it.


Still, rabies can, under certain conditions, approximate some of the conditions of the zombie lust for brains. The rabies virus causes massive inflammation, or swelling, of the brain, and it's most often transmitted by bites from infected animals. About 55,000 people die annually from rabies, with almost all of these deaths occurring in Asia and Africa. Although vaccines do exist (indeed, it was Louis Pasteur's successful treatment of a rabies-infected child that brought us into the modern age of vaccinations), they have to be administered before the onset of symptoms if the patient is to survive.


Again, the symptoms of rabies sound rather like those of the walking dead: full or partial paralysis, mental impairment, agitation and strange behavior, mania, and finally delirium. It takes a bit of cherry-picking of symptoms, but one could put together a rabies patient with an inability to think clearly or communicate, difficulty walking, and manic aggression that takes the form of frequent attacks on humans.

Although such a zombie-esque sufferer is medically possible, such a hypothetical patient is apparently really, really unlikely. Human-to-human transmission of rabies is incredibly rare, and it almost always occurs thanks to insufficient background tests before organ transplants. (I kinda hate to say it, but just like that one episode of Scrubs.)


So there aren't very many, if any, people going around biting other people. But, to be fair, rabid animals do act a whole lot like extras in 28 Days Later, with uncharacteristic behavior, aggression leading to attacks, and an apparent loss of all reason. Until human rabies manifests itself more like animal rabies - and it's not something I'm exactly hoping for - that's about the closest we're getting to real world brain-hungry zombies.

Necrosis

Those of you who are up on your Greek roots already know where we're going with one: necrosis is death, specifically those of individual groups of cells before the organism as a whole dies. This isn't technically a disease but rather a condition with a lot of different possible causes. Cancer, poison, injury, and infection are all possible causes of premature cell death.


If we're being super-literal about what the walking dead really are, then a patient with necrotic tissue is maybe the closest equivalent. After all, a patient suffering from necrosis technically is partially dead, albeit still very much alive in all the important areas (the brain, the heart, and the rest of the vital organs, for a start) that we generally associate with the living.


Whatever its external (or, in the case of cancer or infarction, internal but extraordinary) cause, necrosis triggers a series of event that can lead to even greater negative effects outside the affected area. The dead tissue stops sending signals to the nervous system, and necrotic cells can release dangerous chemicals that hurt nearby, still healthy cells. If the lysosome membrane inside the cells is damaged, enzymes can be released that can also harm surrounding cells.

This chain reaction can cause the necrosis to spread (and if it spreads over a great enough area, it becomes gangrene) and can ultimately be fatal. The only way to cure the condition is through a process known as debridement, which is simply the removal of necrotic tissue. If the dead area is too large, this may require amputation.


If there is any sort of bright side to all this - and I'm not sure there is, but I'll put my Pollyanna hat on and try my best - at least necrosis isn't contagious, meaning it's not the sort of thing that could spur a faux-zombie outbreak. Of course, a sudden wave of hyper-aggressive, necrosis-spreading spiders or snakes? That might be another matter entirely.

Dysarthria

Let's take a bit of a break and talk about something relatively less serious. ("Relatively" being very much the key word there.) We've talked about possible causes of zombie-like trances, cell death, and hyper-aggression. What about something a little more innocuous, like the iconic moans and grunts of the oncoming zombie horde? What could cause that?


Well, the best real-world equivalent is probably dysarthria, which is a disorder affecting the motor controls of human speech. Dysarthria is particularly appropriate because it's neurological in its origins, which ties in with the brain-based aspects of zombie lore. There are a lot of different causes of dysarthric speech, but all are characterized by a malfunction in the nervous system that makes it difficult to control the tongue, lips, throat, or lungs.


This in turn causes difficulty in articulation, which can take the form (among many possible manifestations) of an inability to communicate in more than unintelligible noises. The condition can be brought on by traumatic brain injury, metabolic diseases like Lou Gehrig's or Parkinson's, or a stroke, all of which lead to a loss of control over the vocal muscles. Possible affected areas include the ability to regulate the volume of speech, the ability to create the proper inflection, and, most importantly for our purposes, the ability to create the correct sounds of speech.

To be sure, in and of itself dysarthria is not a particularly zombie-ish condition. However, coupled with any of the other diseases on this list, it gets you frighteningly close to a real approximation of the sight and sound of the walking dead.


Leprosy

Both zombie folklore and leprosy have a long, long history. Armies of the flesh-eating undead can be traced all the way back to the roughly tenth century BCE Akkadian work The Epic of Gilgamesh, which drew on earlier Sumerian mythology and was one of the first substantial written works in human history. Cases of leprosy have been reported going back some four thousand years throughout Eurasia and northern Africa, including China, India, and Egypt. Considering a common feature of zombies is their rotting flesh and decaying body parts, it would seem like leprosy and its similar-sounding symptoms would be a natural inspiration for such stories.


Well...sort of. The truth is (as usual) rather more complicated. First of all, it's a myth that leprosy causes body parts to rot away and fall off - indeed, there really aren't any diseases that can actually make limbs fall off (although, as discussed earlier, necrosis can necessitate the amputation of dead limbs). Leprosy can cause damage and numbness in its victims, which could cause a slow, shuffling walk that might have inspired the gait that we associate with zombies. The main external symptom of leprosy is the outbreak of extensive skin lesions, which gives the skin a diseased, decaying appearance not unlike that of the common conceptions of zombies.


Fortunately, leprosy is pretty much under control at this point, certainly compared to sleeping sickness. Over 95% of people are naturally immune to the disease, and over fifteen million people have been cured of the disease in the last two decades. It's a remarkable turnaround for once of the most feared and stigmatized diseases in human history - indeed, for centuries leprosy evoked the same kind of irrational dread that we might now feel towards the dead rising from the graves en masse, ready to devour our brains.

Although there are definitely a few connections to be drawn between the symptoms of leprosy and the supposed traits of zombies, maybe the most fascinating overlap can be found in the story recounted in John Tayman's 2007 book The Colony: The Harrowing True Story of the Exiles of Molokai. As the Zombie Research Society reports, the book describes how the lepers at the Hawaiian colony were literally treated like they were the walking dead.


The leprosy patients were judged legally dead, their spouses were granted immediate divorces on the grounds they were basically widows anyway, and their wills were executed. The patients were then banished to a remote island where they were left to die, although some survived on the island for decades. This tragic part of Hawaiian history - a story with plenty of echoes elsewhere - is pretty close to how one might expect society would actually treat zombies if they existed.