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Zombie fiction has grown increasingly popular over the last several decades, but the origin of zombie outbreaks varies based on its source material.
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In George A. Romero’s cult classic Night of the Living Dead, anyone who dies turns into reanimated flesh-eating ghouls. In The Walking Dead, the zombie infestation is linked to an airborne virus that infects everyone, slows down decomposition and reactivates a person’s brain stem after they’ve died. A bloodborne virus is so potent in 28 Days Later that just one drop of blood or other bodily fluid is enough to send its victims into a frenzy with uncontrollable rage within mere seconds of being infected. And depending on which of the Resident Evil installments you’ve played, the zombie apocalypse is linked to either a centuries-old fungus or a lab-made virus capable of causing genetic mutations in anyone that comes into contact with it.
After COVID-19, the thought of a widespread pandemic that turns people into zombies is certainly plucked from the deepest corners of our most terrifying nightmares. Scientifically, there’s no such thing as a zombie virus. But zombie narratives are often rooted in scientific truth about how infections spread.
Clinical microbiologist Anisha Misra, PhD, explores the science behind zombie lore and explains why everyone is up in arms right now about the potential dangers of fungal infections.
There’s a long list of zombie myths across multiple mediums. Nearly every one of them is rooted in some scientific truth:
The Last of Us is a video game-turned-HBO show in which a widespread zombie apocalypse is started by cordyceps. In the story, cordyceps is a parasitic fungus that takes over your body and causes you to infect others through fungal spores or mycelia (tendrils).
In reality, cordyceps can’t infect humans but it is a parasitic fungus that infects specific types of insects in specific geographic areas. In fact, there are more than 750 species of cordyceps and each one targets a specific kind of insect. At least two of those species have potential health benefits for humans. But should we be concerned about a potential human fungal infection from cordyceps?
“It’s not likely that cordyceps could jump species and infect a human being in the same way the show entertains us with,” says Dr. Misra. “But it does bring up a valid point: Fungi can develop thermotolerance (resistance to high temperatures) and infect individuals with a higher body temperature.”
In fact, this is already happening. Some fungi like Candida auris are causing new and widespread infections, presumably because global warming, higher temperatures and larger populations of people with compromised immune systems are making that more possible.
“Because these fungal organisms have been able to adapt and cause infections within humans, they’ve become organisms of interest,” adds Dr. Misra.
Resident Evil stands out from other zombie lore because it focuses on the idea that a zombie virus can alter or change your genetic code. These alterations then lead to enormous physical mutations that forever change the person infected.
“This concept is based in truth,” says Dr. Misra. “Viruses have a simple structure, so they can’t reproduce on their own. They rely on the host’s cell machinery in order to reproduce and make more viruses. Some viruses do this by inserting their own DNA or RNA into our human cells.”
But genetic mutations that occur as a result of viruses are far more subtle than zombie fiction will have you believe.
One way to think about it is to imagine each individual cell in your body like a tiny city and the nucleus of each of your cells like its own library that stores books of genetic information (DNA) and functional copies of that information (RNA) that explain how to carry out a number of biological processes.
The goal of any virus is to mass produce copies of itself, but it can’t do that on its own — so it needs to use libraries (your nuclei) to do the work. Viruses do this in a lot of ways, but a specific kind of virus called a retrovirus makes copies of itself by shoving its own genetic code into healthy, bound volumes of DNA. When your cells grow and multiply, this DNA is then copied over into new cells along with fragments of the retrovirus so that it can continue to be copied beyond its original infection.
And sometimes, a retrovirus will infect a germ cell (a cell that develops into a reproductive cell). When that happens, parts of the retroviral DNA can be passed down into the DNA of your offspring. Over time, what started off as a simple virus becomes a part of your actual genetic code. In fact, about 8% of our human genome is made up of inactive virus fragments.
“This has happened over time for millions of years, and there are viruses that infect and integrate into our human genome,” says Dr. Misra.
Some retroviruses that have the capability of altering your genetic code include:
It’s important to note that while these retroviruses have lasting effects, they’re preventable and there are different treatment options for each, including vaccines, antibiotics, pre-exposure prophylaxis (PrEP) and over-the-counter medication that can help with symptoms.
One common thread in all zombie fiction is how infections have an effect on your brain — but are there any real-world infections that impact your ability to think clearly?
Some viruses, bacteria, parasites and fungi have the ability to cause inflammation in and around the brain, especially if they’re left untreated.
When you develop meningitis, the area known as the meninges surrounding your brain and spinal cord becomes inflamed. This can lead to confusion, lack of energy, lack of appetite, hallucinations and difficulty with focus and attention.
You can also develop encephalitis, which is similar but causes inflammation in your brain itself. Encephalitis causes changes in behavior, confusion, difficulty speaking or moving, issues with memory, and seizures. The most common cause of encephalitis are viruses that include:
“With encephalitis, symptoms tend to start out pretty mild but they can result in confusion, agitation or hallucinations,” states Dr. Misra. “It can also result in a loss of consciousness, including comas because it affects a specific region in your brain.”
The rabies virus (RABV) is another that’s perhaps the most well-known virus to cause behavioral changes and lead to a fatal infection. This virus spreads through a break in your skin that comes into contact with the spit (saliva) of an infected animal, most often from a bite. Once bitten, you can get a vaccine that stops the infection. But if you’re not treated right away, you could experience symptoms that include:
“Once the rabies virus reaches the brain, your symptoms progress through cerebral dysfunction,” explains Dr. Misra. “Once these clinical signs appear, the disease is nearly always fatal.”
An infection caused by a zombie virus usually leads to increased decay and cell death, much like necrotizing fasciitis. In reality, necrotizing fasciitis is a flesh-eating disease that affects your muscle fascia (thin, connective tissue) under the surface of your skin and causes those tissues to die. Most often, this condition is caused by bacteria that get into your body through a break in your skin like a cut, scrape, burn or other wound.
“These organisms spread very quickly,” warns Dr. Misra. “If you’re not treated quickly with antibiotics, or the infected tissue isn’t removed, you can go into a toxic shock-like syndrome that can result in sepsis and organ failure.”
When you first develop this condition, the affected area may feel hot to the touch or have a burning sensation. You then develop pustules that start necrotizing or dying off, causing your skin to turn black.
“Necrotizing fasciitis doesn’t spread from human-to-human skin contact, but through inoculation of your skin,” says Dr. Misra. “That means that if you touch the infected area, you likely won’t contract the infection. For the infection to spread from one person to another, the bacteria have to enter your body through an open wound.”
Zombie fans familiar with 28 Days Later will remember an iconic scene in which one person gets infected by a single drop of blood that falls into their eye — and that’s all it takes to infect them within seconds.
“Is it possible a single drop of blood could cause infection? Yes, but a lot of things would have to align in order for that to happen,” says Dr. Misra. “That drop of blood would have to have enough infectious particles in it in order to infect another person and the infection wouldn’t spread that quickly. But if that person was immunocompromised, their chances of getting infected would be even higher.”
That said, anytime healthcare providers are working with blood or other body fluid, they take necessary precautions by wearing gloves, glasses, gowns and other protective equipment to protect from possible infection. If someone were to get exposed, they’re given immediate treatment in the form of antibiotics or vaccines and tested for several potential infections like hepatitis B and HIV.
“It depends on how quickly you act and on the pathogen itself,” Dr. Misra continues, “but a drop of blood is normally not enough to cause an infection.”
You can’t have zombies if you can’t bring back the dead. While that’s scientifically impossible (at least for now) one study is pushing the boundaries between life and death.
Yale researchers were able to restore function to cells across multiple organs in pigs that were dead for more than one hour in an effort to revive those organs after death. The results are promising, particularly in the area of organ transplants.
“Currently, we transplant organs after brain death because we’re still able to circulate blood to keep those organs healthy,” Dr. Misra explains. “But in cases of sudden cardiac arrest (your heart stops beating), blood stops flowing which causes our organs to start deteriorating. If someone dies of a cardiorespiratory death, we’re not able to use all their organs because by the time we’re able to collect them, they’ve gone through some sort of cell death.”
Far more research is needed before this could ever be used in a clinical human trial. But the mere thought of reanimating the dead is compelling, even when you consider its uses outside of zombie fiction.
“There was even some cell regeneration that took place, which means we could be collecting organs from these cardiorespiratory deaths and increase our pool up to 20% for transferrable hearts just by using this method,” adds Dr. Misra.
But until more research is done, and perhaps policies in place to protect human life, this study only hints at what’s possible when we leave behind fiction and begin embracing scientific exploration.
“Like anything crossing these thresholds of life and death, it brings up a lot of ethical questions into play as to when we would be able to declare someone dead in order to then be able to transplant their organs,” notes Dr. Misra. “But it’s an extremely interesting study because of the magnitude of its potential for humans.”
Regardless of the origin of infection and how these scientific truths inspire a multitude of zombie stories, it’s important to remember that a lot of infections caused by viruses, bacteria, parasites and fungi are preventable and treatable if caught early enough. If you ever do experience symptoms or are concerned, talk to a healthcare provider instead of waiting it out. It’s always better to be safe than sorry.
Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy
A clinical microbiologist explores the science behind popular zombie lore.