There are literally thousands of books and movies featuring zombies. In books and movies, human beings can be transformed by disease or curse into mindless horrors driven either to eat your brains, or – worse – to infect you so that you turn into a zombie yourself. The zombie theme is woven deeply into our culture.
We seem strangely preoccupied with these monstrously “undead” creatures. Why does this theme turn up so often in our fiction? Could it be that our obsession with fictional zombies reflects a real-world concern? Do zombies, or something like zombies, actually exist in nature? Surprisingly, the answer is yes. It all has to do with the influence of parasites. And that’s the topic of this blog.
Researchers have determined that parasites turn their hosts into zombies whenever they can. They do it for the most understandable of reasons – zombifying their hosts helps them to survive and reproduce. Parasites evolve ways to redirect host behavior so that it favors the parasites’ own reproduction. Almost all parasites cause their hosts to act in specific ways that support the parasite’s lifecycle.
Zombie mice
One better-known example – one that has been in the news a lot lately – is the parasite toxoplasma gondii. T. gondii is known to change the behavior of infected mice to make them more likely to be eaten by cats, their natural predators. The “zombified” mice are influenced to act in ways that directly threaten the mouse’s own interests.
Normal mice are terrified when they detect the odor of cat urine. Cats are their natural predators. So in response to the odor normal mice either hunker down out of sight, or run away as far and as fast as they can. But mice infected with T. gondii act very differently. They don’t seem to be afraid of the smell. Instead, they appear intrigued by it, and sometimes follow the odor to its source. In addition, in a very un-mouselike way they become more active, and so draw attention to themselves.
These behavioral changes clearly are not in the mouse’s interest; they greatly increase the odds that it will be eaten by a cat. But their altered behavior does suit the needs of the parasite. That’s because in order to complete its reproductive cycle this particular parasite must transfer itself from the body of the mouse to the body of a cat. So the parasite has evolved the means of converting the mouse into a zombie assistant that helps it reach this goal.
It ain’t magic
How in the world could T. gondii make a zombie out of a mouse? Though it might seem like magic, it’s really biochemistry.
T. gondii forms cysts that nestle inside the mouse’s brain and excrete chemicals that redirect brain activity in a way that favors parasite transmission.
Responding to those chemicals, the zombie mouse is attracted to the smell of cats, is less fearful, and is more likely to move around actively. The upshot is that T. gondii changes the mouse into a different creature – a zombie that serves the interests of the parasite.
Although this zombie story is interesting, it may not at first seem relevant to our own lives. But it is indeed relevant. Unfortunately, a significant percentage of the humans on the face of this earth are also infected with T. gondii. The reason we are vulnerable to infection is that some of our closest animal relatives are the prey of big cats. In Africa, monkeys and apes are frequently eaten by large felines. In fact, primates constitute a large part of their diets.
One group of researchers looked at our closest relative, the chimpanzee. They found that infection does change a chimp’s behavior. For example, “Toxoplasma-infected animals lost their innate aversion toward the urine of leopards… their only natural predator.” Chimp zombies. 1
Your zombie neighbor
Humans are primates. So humans are subject to T. gondii infection. Studies confirm that a sizable percentage of the world’s population – about one third to one half – harbor the parasite. In fact, T. gondii has inserted behavior-manipulating cysts into the brains of up to half the humans on planet earth. Incidence varies with the country, ranging from 20% to 80%. The rate in the US is toward the lower end of the range. 2 But that’s still a minimum of one out of every five citizens.
It’s natural to wonder whether T. gondii changes human behavior, as with its other hosts? Once again, the answer is yes. Because our biology is pretty much the same as the rest of the primate group, it makes sense that the parasite would affect our bodies and our behavior in a similar way.
Research confirms that T. gondii does indeed impact many aspects of human life – an eye-opening reality that we are just beginning to appreciate.
In what way does T. gondii change us? The alterations in behavior provoked by T. gondii infection are in fact numerous, and are substantially different for men and women. 3 Personality changes are detectable with standardized tests such as Cattell’s 16PF. Researcher Jaroslov Flegr undertook a review of 11 studies, and concluded that
Consistent and significant differences in Cattell’s personality factors were found between Toxoplasma-infected and uninfected subjects in 9 of 11 studies, and these differences were not the same for men and women… The personality of infected men showed lower superego strength (rule consciousness) and higher vigilance (factors G and L on Cattell’s 16PF). Thus, the men were more likely to disregard rules and were more expedient, suspicious, jealous, and dogmatic. The personality of infected women, by contrast, showed higher warmth and higher superego strength (factors A and G on Cattell’s 16PF), suggesting that they were more warm hearted, outgoing, conscientious, persistent, and moralistic. Both men and women had significantly higher apprehension (factor O) compared with the uninfected controls. 4
Do these personality differences have any real-world consequences? Again, yes. Infection makes men more willing to take risks; studies indicate that infected men are nearly three times more likely than non-infected men to be involved in automobile accidents. 5 This effect is almost completely reversed in women, whose risk-taking declines.
Parasites. Zombies. It’s important to understand that the “zombie effect” is not unique to T. gondii. Every parasite will, if it is able to do so, influence the behavior of its host to favor its own reproduction.
Is every parasite that infects a host a different animal, a different species? No, and this is really curious. Some parasites spontaneously develop from within the host. Cancer, for example, begins when one of the cells in an animal abandons the animal’s interests and begins acting in its own interest instead. The cancerous cell is a cell that has gone rogue. Its own reproduction has become an end in itself. It becomes a tumor. Bizarrely, cancer can be interpreted as a parasite that has emerged from within.
Zombifying habits
Now here’s something really interesting: Habits can go rogue in the same way that the cells of our bodies can turn cancerous, coming to serve their own interests rather than ours.
If we ask ourselves what human behaviors seem most zombie-like, we will invariably turn our attention to addictions. Addictive habits bear a striking resemblance to the zombie behavior of parasitized animals. The zombie-like addictive pattern doesn’t serve the interests of the addict. In fact it often destroys the addict. But the pattern does serve to perpetuate the addiction itself. It’s my belief that addictions can be interpreted as cancerous habits, parasitic behavioral patterns, habits gone rogue.
The zombifying influence of parasites is one of the topics in my latest book, Rogue Habits. If this strikes you as an interesting topic, you can read more about it in a somewhat more formal paper “Zombies Among Us.”
References
- Poirotte C, Kappeler PM, Ngoubangoye B, et al. Morbid attraction to leopard urine in toxoplasma-infected chimpanzees. Current Biology, 2016, 26, 3, R98-R99. Page R98.
- Henriquez S, Brett R, Alexander J, Pratt J, Roberts C. Neuropsychiatric disease and toxoplasma gondii infection. Neuroimmunomodulation. 2009, 16, 2, 122-33. Page 122.
- Flegr et al. Induction of changes in human behavior by the parasite protozoan toxoplasma gondii. Parasitology, 1996, 113, 49-54.
- Flegr J. Effects of Toxoplasma on Human Behavior. Schizophrenia Bulletin, 2007, 33, 3, 757–760. Available online at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2526142/
- Flegr J et al. Increased incidence of traffic accidents in toxoplasma-infected military drivers and protective RhD molecule revealed by a large-scale prospective cohort study. Biomed Central Infectious Diseases, 2009, 9, 72.