There’s nothing scarier than a zombie outbreak, as the lurching, swarming masses on season six of AMC’s The Walking Dead reminds us. The possibility that people could collectively be transformed into monstrous, empty shells of their former selves taps into our fears of the unknown and uncontrollable.
Zombies also offer us an outlet to examine dramatic epidemics without directly confronting our vulnerability to contagious diseases. In modern-day zombie lore, the cause of the dreaded brain-eating disease is usually an infection. A bacterium, virus, fungus, or even a deformed protein can be transmitted from person to person to cause “zombification.” These premises may remind us of many real-life infectious epidemics of recent decades, from SARS to “bird flu” to the recent (and ongoing) Ebola epidemic.
But suspending disbelief on the whole bringing-back-the-dead thing, just how realistic are these zombie outbreaks? As a professor specializing in infectious diseases, I felt duty-bound to rate them using a scoring system of 0 (never going to happen) to 10 (completely plausible scenario). Warning: possible spoilers ahead.
The Walking Dead
On The Walking Dead, the zombie epidemic is terrifyingly inescapable. At the end of season two, former sheriff Rick Grimes informs a group of fellow survivors that that they’re all infected with the zombie agent. No matter how they die, they’ll become zombies.
But the big mystery is where this infection came from. Early in the series, a scientist shows an image of a virus-y looking agent to survivors. The spinoff series Fear the Walking Dead offers a few clues about the beginnings of the epidemic. There are reports of a “flu” outbreak, and people are becoming ill. Absenteeism is up at the school where the two main characters, Madison and Travis, work.
Working with the little we know about the infection, how it could have left no one in the human population untouched?
Common ways that germs can be transmitted quickly to a large swath of the population include food, water, and air. However, it would be very difficult for any one of these substances to become contaminated at a single point in time so that everyone in a country (or even around the entire globe?) could be infected all at once.
We don’t all eat the same food or use the same water supplies. Airborne release of a contaminant could be coordinated in a number of large cities at once. But it would take the contaminant considerably more time to spread to rural areas.
Even if this could be accomplished, you’d still expect some small percentage of the population to be resistant to the virus and avoid becoming infected. Yet we have yet to see a single person in The Walking Dead universe who doesn’t come back as a zombie after death, with the exception of those whose brains were already destroyed. Realism score: 4 out of 10.
28 Days Later
In 28 Days Later, the source of the outbreak is a medical research lab that’s been conducting experiments on chimpanzees infected with the genetically-modified “rage” virus.
In this case, the infection spreads directly via bite. Because infected people now possess heightened speed and strength, they soon overtake the human population.
These are living zombies. There’s no death or reanimation involved, which ups the realism factor. However, the incubation period of the virus—the time between being bitten and showing symptoms—is almost immediate. Less than a minute passes between bite and zombification.
In the real world, microbes that have to replicate in the body take a minimum of 24 hours to cause disease. Most take three to four days. And some, such as rabies, can take weeks to months. A practically-instantaneous incubation period is not even close to realistic. Realism score: 5 out of 10.
World War Z
Like 28 Days Later, the movie version of World War Z shows humans immediately developing symptoms after a zombie bite.
While Gerry Lane (Brad Pitt) travels around the world in search of the outbreak’s origin, he observes that zombies seem to people who are already sick. He uses this information to develop a “camouflage” when he is trapped in a zombie-filled microbiology laboratory. He randomly chooses a stock of a microbe and injects himself with it. This allows him to escape from the zombie now that he is ill and “invisible” and share his findings with the world.
In the book version, the infection (caused by the fictional Solanum virus) takes much more time to develop. The outbreak is stopped by sheer military force and the creation of zombie-specific weapons such as the “lobotomizer.”
So how does all this fare on the realism scale? There is some evidence that we can use one microbe to fight another, as in the movie version. Antibiotics, for example, are just chemicals taken from one species of bacteria or fungi and harnessed by humans to fight off others.
However, that’s not exactly what’s happening in World War Z. The sick individuals must be giving off some kind of scent, perhaps a pheromone, that renders them immediately “invisible” to the zombies.
Theoretically, an infection may have this effect on humans. But not nearly enough time had elapsed between Lane’s injection of the microbe and his newfound invisibility to allow the host’s immune system to respond. Military force seems a more likely method for eradicating zombies, even if other mathematical models have shown that’s unlikely to help very much. Realism score: book, 6 out of 10. Film: 0 out of 10.
Resident Evil series
In the Resident Evil series, adapted from a video game, the bioweapons-developing Umbrella Corporation has created and released the T-virus, which reanimates the dead and turns animals into exotic mutants. These are not films that care about scientific explanations, so let’s just move on. Realism score: -20 out of 10.
The Last of Us
The Last of Us, a video game that’s now being adapted for the silver screen, has the most unique explanation for a zombie outbreak.
In the game, the zombie pathogen is a fungus rather than a virus. Humanity has been decimated by a mutated version of the Cordyceps fungus, which grows in the brain and turns infected humans into progressively more dangerous zombies. Eventually, the infected host seeks out a dark, warm place in which to die and release spores, which further spreads the disease.
This sounds far-fetched. But much of the set-up is based on a very similar fungal infection in a wide variety of insect species. When the insects are infected with the fungus, they become “zombies” under the control of the fungal pathogen. The fungus essentially takes over the host’s nervous system, leaving the host as little more than a spore-producing husk.
While insects don’t become aggressive, they do seek out environments that will maximize the spread of spores to others in the population once they die and the fungus bursts from their body. These types of fungi are not known to infect humans, but there is some research that suggests other pathogens (such as the cat parasite Toxoplasma) may affect our behavior. So The Last of Us wins the prize for realism. Realism score: 7 out of 10.
The upshot is that there’s no need to fear an undead apocalypse—at least not of the nature most commonly presented in popular culture. That said, these infection scenarios should at the very least get people thinking about the importance of preparing a “bug out” bag and stocking up on supplies of food and water for natural disasters. And one last thing: get your flu shot while you’re at it.