The microbiology of zombies, part III: “We’re all infected”

Warning: here be spoilers

In many latter-day zombie movies, books, and TV shows, zombie-ism has a biological cause. In 28 Days Later, the infection is caused by the “Rage” virus, which escaped from a lab when animal rights activists break in and release a group of infected chimpanzees. Of course, one of the animals promptly bites one of its “liberators,” and the infection spreads rapidly throughout Great Britain. In Zombieland, it’s a mutated form of “mad cow” disease. The Crazies, it’s the Trixie virus; World War Z, the Solanum virus; Resident Evil, the T virus. I could go on and on. Zombie causation has clearly evolved from the early days of radiation or curses, and has become a biological phenomenon in most modern zombie tales.

The Walking Dead is no exception. Though the claim is made in season 1, episode 6 (“TS-19”) that the outbreak could be caused by just about anything–bacteria, virus, parasite, act of God–I call shenanigans. In the previous episode (“Wildfire”), Jenner, the CDC scientist, is processing tissue taken from Test Subject 19, and the visualization under his microscope looks very viral. Of course, take this with a few pounds of salt, since he’s using a light microscope and can also see the nice alpha-helical DNA strains within the pathogen (in real life, things just don’t look like this) and unless you’re one of the giant viruses, you can’t see viruses, much less DNA, under the microscope Jenner uses anyway. But still, it looks pretty viral-y to me, which is why I typically refer to it that way:

screenshot wildfire virus

Microbial zombification makes sense in today’s culture. My colleague Brooks Landon notes: “…zombies represent a better monster for the modern, post-9/11 world. They provide a release for feelings of being overwhelmed by abstract and intractable events like global economic crises, terrorism, and pandemics.” In the past decade or so, we’ve seen the emergence of SARS, multiple outbreaks of influenza including a new pandemic strain, the continuing HIV crisis, Nipah, Hendra, more Ebola, just to name a handful. Infectious diseases are commonly in the news, and many times are unfortunately over-hyped, leading to a collective nervousness of all things microbial.

The infected zombie is further boosted by a number of recent studies, largely in insects, that demonstrate a type of pathogen-directed “mind control:” zombie ants, zombie grasshoppers, and zombie cockroaches, just to name a few. A recent video game has exploited the ant fungus idea, mutating it into a form that infects humans. Even rodents (and possibly humans) can have their behavior apparently influenced by a parasite called Toxoplasma gondii, which makes rodents lose their fear of cat scents and may influence the development of schizophrenia in humans, or more controversially, even affect sexual inhibitions. If germs are already controlling our minds–why couldn’t they turn us into zombies?

And certainly, there are some candidate microbes which could, in theory, cause at least the “living” form of zombie-ism, even if they couldn’t necessarily raise you from the dead. The Trixie virus, for example, is supposed to be a weaponized rhabdovirus–the family of viruses that includes rabies. Rabies virus infection certainly causes aggression and biting. The virus is spread via saliva, so biting is the main way it is transmitted between animals. In a recent book, Rabid, the authors trace rabies through history, and note that it may be at the root of many zombie (and vampire) tales. Rabies can also hide out in the body for awhile before showing symptoms, as the virus travels up the nerves toward the brain. This is why a bite near the head progresses to symptoms much faster than, say, one to the foot. Typical time from bite to symptoms is in the neighborhood of 6 weeks, depending on the location of the bite and dose of virus one receives, but extreme cases have been documented, with symptoms not showing up for as long as 8 years. And, like has been done on The Walking Dead, one of the ways that bitten victims would try to avoid symptoms would be to cut off the affected limb before the infection spread. (Ouch).

Could something like the “we’re all infected” scenario used in the Walking Dead occur in real life? Maybe. With rabies, victims could appear physically fine for months to years. Even more extreme, there are a number of germs which can remain with people throughout their entire life. The virus that causes chicken pox, for example, doesn’t ever really go away. Your body fights it off enough to keep it in check after the initial rash, but it hides out  in your nerves and can come back in later years as shingles. Other herpes family viruses have a similar lifestyle: symptoms can come and go, but the virus never really leaves. The human papilloma virus (HPV) can also persist for years in some people (most infected people appear to clear this one, though). A bacterium called Helicobacter pylori can live very happily in a person’s stomach–sometimes causing ulcers, but going completely undetected and causing no symptoms in most people. And of course, HIV, which does not go away except in a few notable and high-profile cases. So the concept is, as they say, biologically plausible.

The problem isn’t necessarily with the microbiology, then, but with the epidemiology. How did everyone get infected so quickly? We know that the plague took an incredibly short time to spread (Jenner says less than 200 days in the first season, and “less than 63 days” since it went pandemic)–but how? That’s a missing link in this scenario. We know the pathogen can certainly be spread by bites and then cause zombification that way, but other forms of inoculation (such as getting sprayed in the eyes or nose with zombie blood) don’t seem to have that effect. Is it in the water? If so, that would be some damn rapid spread, since early on Jenner noted that this appeared to be a true pandemic–present around the world. How would that happen?

In the air? Possibly, but even most airborne microbes don’t hang out indefinitely; they’re dispersed by wind to levels below those able to cause infection, or killed by sunlight or other environmental conditions. So even if you had a herpes- or HIV-like virus that could hide out in the body for an extended period of time without causing symptoms, how did *everyone* get it in such a short timeframe? Some scenarios in other books and movies put the blame on bioterrorism. The above-mentioned Trixie virus, for example, was a bioweapon which was only accidentally released when the plane carrying it crashed. Spread of Trixie in the movie ended up being only local, but transmission beyond that is hinted at the end. A true bioterrorist attack could, theoretically, account for simultaneous outbreaks all over the world.

Finally, though the “infected zombie” is now the most common type, it should be noted that this isn’t really new. George Romero, widely recognized as the grandfather of the modern zombie, acknowledges that he “ripped off” his idea for Night of the Living Dead from Richard Matheson’s I am Legend–a vampire story from 1954. The cause of that vampirism?

Bacillus vampiris–a bacterium.


See also:

Part I: the microbiology of zombies

Part II: ineffective treatments and how not to survive the apocalypse

Part IV: hidden infections

The microbiology of zombies, part II: ineffective treatments and how not to survive the apocalypse

(Spoilers. And things.)

After the start of season 4 of the Walking Dead and the introduction of a new nemesis: a fast-spreading, deadly infectious disease that seems to be a strain of influenza, I was looking forward to the plot arc of this season.

And then episode 3, “Isolation”, happened. From an infectious disease standpoint, I say, bah.

At the end of the previous episode, “Infected”, the group had decided to lock up anyone who was showing signs of the infectious disease within the death row cellblock, so that they would not further spread the disease, and to put the children and elderly (as the most vulnerable population) in another area to keep them safe from the infection. Quickly it was seen that this wasn’t working well, as people were becoming sick all over and more and more were moving into the isolation cellblock.

So, a council meeting was called of the leaders of the group. One of the decisions which was made, on the advice of Hershel the veterinarian, was to try to scavenge supplies from a college of veterinary medicine approximately 50 miles away from their location at the prison. What supplies?


For the micro people reading, you’ll see why my rage started boiling a bit at this point. Hershel was the one who’d suggested this was an influenza outbreak (and therefore, caused by a virus) in the prior episode. He is familiar with the disease (and there is another physician, Dr. Subramanian, who has been treating the ill and has seen the rapid course of the disease–of course, he is now sick himself). It is true that influenza can be complicated by a secondary bacterial infection: that those sick with the flu could develop pneumonia due to Staphylococcus aureus or other bacteria, and that these bacterial infections would respond to antibiotic treatment. But, when the course of disease is as rapid as it appears to be during this outbreak, it’s more likely that people are dying from primary influenza infections, which are most certainly NOT treatable with antibiotics. There are antiviral drugs that can treat influenza infections if given early in the disease course (such as oseltamivir or zanamivir ), but I think the odds of those being stocked at a veterinary school would be pretty slim.

So, rather than at least try for some kind of medically plausible scenario (is that really too much to ask?), Daryl, Michonne, Tyreese and Bob the medic take off in search of completely ineffective antibiotics,and run into an enormous zombie horde on the way. Hershel, in the interim, leaves the relative safety of the prison (he was ensconced with the children as a “high risk” individual) and wanders out into the woods to pick berries and leaves to brew elderberry tea. A folk remedy, there are a few peer-reviewed publications which suggest that elderberries or elder flower might have some properties that do work to treat influenza, so at least here Hershel is, well, sucking somewhat less here when it comes to proposing medical interventions to help those suffering than he did with his terrible antibiotics idea.

Hershel does end up with his tea, taking it into the isolation cell block and distributing it to the infected. This includes Dr. Subramanian, who repays the favor by coughing bloody sputum all over Hershel’s face. (Seriously, he doesn’t even know how to cough into his elbow? Even the little girl talking to Carol did that correctly).

From the previews of next week’s episode, “Indifference”, it appears there will be more searches for drugs, while presumably the horde advances toward the prison. I anticipate a miracle cure of some kind for Glenn at the least, but remain annoyed that the writers are touting antibiotics for a viral infection when flu season is upon us.

See also:

Part I: the microbiology of zombies

Part III: “We’re all infected”

Part IV: hidden infections

The microbiology of zombies, part I

(Spoilers below!)

For Walking Dead fans and readers of this blog, you probably know why I was all excited about some of the plot elements that have been included thus far this season: possible zoonotic disease, and in particular, a potential influenza outbreak that may have originated in pigs. I muse about this and other infections in an article for, and will have additional thoughts about zombies and infectious disease more generally in the coming days.

See also:

Part II: ineffective treatments and how not to survive the apocalypse

Part III: “We’re all infected”

Part IV: hidden infections