Back in 2011, the United States Center for Disease Control (CDC) released an official blog post on preparing for a “zombie apocalypse,” as a metaphor for any kind of natural disaster or disease outbreak. This is all well and good to placate the public, but if you look at the qualities of zombies in various forms of media, from the initial cause of zombification, to the spread of the “disease,” to the manifestation of the “zombie state” you see that the CDC isn’t being ridiculous.

If we want to classify how close a real-life disease is to a state of zombification, we need to get a diagnostic checklist of what constitutes a zombie. By observing some common patterns from depictions of zombies in popular media, we can gather four general zombie characteristics:

1) Mental/emotional/intellectual degradation: The intellectual level of a zombie has completely changed, or is no longer present. There are little to no thoughts or feelings reminiscent of the person they once were. The zombie “entity” has completely taken over.

2) Bodily degradation: Zombies ooze. There is a breakdown of circulatory, respiratory, and gastrointestinal systems. The skin and muscles may fall away, and the body may show extensive bleeding and/or decay.

3) Aggressive compulsion to feed on and/or infect others: Zombies tend to feed on the living, and a side effect of this is that the zombification state is infectious. Being fed on means that you will eventually become a zombie, and will crave living flesh.

4) Persistence: Unless the head of the zombie is removed, it will continue to do its zombie thing. There is no discernable end to a zombie lifespan. Sometimes it is implied that zombies can starve if deprived of living flesh, but that’s not always consistent.

Armed with a checklist, let’s look at a few real-world afflictions to see if they qualify as a zombie-like condition.

Zombie-like condition #1

Infection by Cordyceps fungus typically occurs with a number of different insects; the most well-studied is the interaction between Ophiocordyceps unilateralis and tropical ants. In the tropics, ants living on tree canopies often descend down into the understory for foraging where O. unilateralis spores are present. After the fungus colonizes the brain, the “zombie” ant leaves the nest and wanders to the understory, where it bites down on a leaf vein and dies.1 Atrophy of the ant’s jaw ensures that the cadaver will continue to cling to the leaf without falling. The comparatively humid microclimate of the understory is an excellent condition for the fungus to grow out of the cadaver, culminating in the gruesome sprouting of a mushroom out the back of the head (Google this. It’s amazing) and dissemination of spores to infect other hapless ants.

Can Cordyceps infection be a zombifying disease? Let’s weigh it against the checklist:

1) Mental/intellectual degradation: Yes. Cordyceps infects the brain and guides the ant’s actions to facilitate dissemination of spores

2) Bodily degradation: Not really. Zombie ants do a zombie ant-like stagger, but their appearance is normal.

3) Compulsion to infect others: Sure, see #1. The fungus “drives” the ant to a location that is favorable for the dissemination of a maximum amount of spores. More spores leads to more zombie ants.

4) Persistence: No. Zombie ants die within just a few days.

Why have we not had a Cordyceps zombie apocalypse yet? The host range is limited to insects living in tropical climates.

Could we have a Cordyceps zombie apocalypse? If a host shift occurred, then it might happen. Pathogens can and do shift hosts through evolutionary processes, with avian flu as a big example.

Zombie score: 2/4 —with a big hypothetical attached.

Zombie-like condition #2

Rabies is a neuroinflammatory disease caused by a virus (lyssavirus, aptly named from the Greek goddess of rage). We recognize rabies as the “mad dog” disease; infected animals behave aggressively and can transmit the virus through biting or scratching victims.

Can we refer to rabid individuals as zombies?

1) Mental/intellectual degradation: Big check. Infected animals are confused, paranoid, and prone to violent interactions. They are incredibly dangerous.

2) Bodily degradation: No. Rabid animals “foam at the mouth” due to an inability to swallow, but that’s really the extent to which they ooze.

3) Compulsion to infect others: Absolutely. Rabid animals compulsively attack and bite healthy individuals, and the virus is transmitted via saliva

4) Persistence: No. Rabid animals usually die within a few days.

Why has a rabid zombie apocalypse not yet occurred? Rabies can be controlled through vaccination programs, education, and access to immunoglobulin if there is a chance that exposure has occurred. Another point to consider is the relatively short infectious period, which limits the number of animals exposed by one infected individual before it dies.

How could a rabid zombie apocalypse occur? First, removal of vaccination and post-exposure treatment programs, as rabies is a significant problem for parts of the world where feral animals are common, and there is less access to costly preventative medicine and treatments.2 Second, increase the probability of infection by lengthening the infectious period and increasing the number of animals attacked and infected.

Zombie score: 2/4. Rabies is terrifying. Vaccinate your pets please!

Zombie-like disease #3

Ebola is a viral hemorrhagic fever spread by fluids and starts off with fever-like symptoms. Usually within 48 hours afflicted individuals show petachiae and decreased clotting of blood, which then turns into coughing/vomiting up blood, bleeding into the gastrointestinal tract, and bleeding into the whites of eyes. Death occurs one to two weeks later typically from low blood pressure.

Do we think of Ebola as a zombifying disease? Let’s weigh it against the checklist:

1) Mental/intellectual degradation: No. The brain remains unaffected.

2) Bodily degradation: Yes, absolutely. The Ebola virus can infect a variety of tissues, which results in multiple organ failure and overwhelming blood vessel leakage.

3) Compulsion to feed on/infect: No. Ebola victims are ill and exhausted.

4) Persistence: Perhaps, but probably not. Victims usually die within one to two weeks. Survivors may harbor the virus for some time afterwards without showing symptoms.

Why hasn’t there been an Ebola zombie apocalypse? Everyone remembers the 2014 outbreak originating in Guinea; appropriate public education, sanitation, and quarantine measures kept the epidemic from spreading further. Probably the best thing from an epidemiological standpoint is that Ebola kills its victims relatively quickly, which limits the number of people in contact that could possibly contract the disease.

Could there be an Ebola zombie apocalypse? Ebola is extremely dangerous, with typical mortality rates ranging from 80-90%.4 To date, there is no vaccination, and treatments have limited efficacy. If, hypothetically, Ebola could be transmitted through the air similar to the common cold, humans infected with Ebola sought to actively infect other humans, and the infected roamed around for a longer amount of time, then yes.

Zombie score: 1/4. As scary as an Ebola epidemic is, the fact that it does not take over your mind disqualifies it from being characterized as a zombifying disease.

Zombie-like disease #4

Methamphetamine addiction is a little different from the aforementioned maladies. Here, the victims take it upon themselves to become infected, per se. Methamphetamine causes considerable stimulation of the central nervous system, resulting in euphoria, heightened self-esteem, and increased alertness. This occurs through a heightened and sustained release of the neurotransmitter dopamine into the extracellular space. Furthermore, methamphetamine blocks the reuptake of dopamine, so levels remain heightened beyond the physiological state. The problem here is that constant, heavy release of dopamine beyond physiologically normal levels is neurotoxic, causing oxidative stress and inflammation. Additionally, chronic methamphetamine use causes long-term and possibly permanent damage to dopamine nerve terminals, so users lack the ability to experience pleasure.3

Does chronic methamphetamine use turn us into zombies?

1) Mental/intellectual degradation: Sure. Chronic methamphetamine users experience paranoia, psychosis, hallucinations, and compulsive and sometimes violent behaviors. However, this behavior depends on the amount of drug in the system.

2) Bodily degradation: Yes. Constant skin picking, weight loss, and dental decay (“meth mouth”).

3) Impulse to infect others: Maybe, but not in the biological sense. Methamphetamine addiction epidemics are more of a social issue.

4) Persistence: Yes, compared to the aforementioned maladies. Methamphetamine addicts live an average of 5 years, and are usually killed by something other than the drug.

Why have we not all become methamphetamine-addicted zombies? Choice is a big factor here. If we wanted to, then absolutely!

Could we all become methamphetamine-addicted zombies? Supposedly, if there was some situation in which an evil-doing entity put it into the water supply. But, that’s some conspiracy theory-level thinking.

Zombie score: 3.5/4. Although this may be debatable due to lack of full penetrance and non-biological attributes.

None of these diseases perfectly match the exact combination of attributes to be a zombie-like disease, but hypothetically, if certain attributes of each of these diseases were combined, then we could quite possibly meet the optimal conditions for a zombie apocalypse. This is commonly the inspiration for recent zombie films. For example, the zombies in the film 28 Days Later suffer from what looks like a hybrid of rabies and Ebola, yielding dangerous rage-filed creatures with the appropriate amount of ooze. In the video game The Last of Us, the zombies are cannibalistic human-mushroom hybrids, having contracted a host-shifted Cordyceps fungus with rabid qualities for that appropriate amount of rage and aggression.

The reality is that we are just one or two hypothetical situations away from a zombie apocalypse. So, my recommendation is to stock up on emergency water, canned food, a good pair of running shoes, a first aid kit and possibly a shotgun (if you are into that sort of thing). Even if a zombie apocalypse does not occur, at least we will be prepared for the next natural disaster.

Good luck.

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1 Hughes et al., 2011. Behavioral mechanisms and morphological symptoms of zombie ants dying from fungal infection. BMC Ecology 11:13

2 WHO Fact Sheet No. 99

3 Falasca, L. et al., 2015. Molecular mechanisms of Ebola virus pathogenesis: focus on cell death. Cell Death and Differentiation 22:1250-1259

4 Rusyniak, D.E., 2011. Neurologic manifestations of chronic methamphetamine abuse. Neuro Clin. 29:641-655