This Is Your Brain On Parasites: How Tiny Creatures Manipulate Our Behavior and Shape Society

by Kathleen McAuliffe

Parasites and hosts have been competing with each other for billions of years. The first bacteria were parasitized by the first viruses. When larger, multicellular life forms emerged, these microbes in turn colonized them. Meanwhile, parasites continued evolving into a menagerie of distinct forms—roundworms, slugs, mites, leeches, lice, and the like. As life grew in size and complexity, natural selection favored parasites that were the best at evading hosts’ defenses, and hosts with the greatest skill in repelling the invaders. Today, virtually every aspect of the human body’s design bears witness to this age-old struggle. Our most visible defense is our skin, which provides a thick barrier to the hordes of microbes that populate its surface. Entry points are especially fiercely guarded: Eyes are bathed in tears that flush out intruders. Ears are lined with hairs to keep out bugs. The nose has a filtration system for screening pathogens out of the air. Invaders that make further inroads will only encounter stiffer resistance. The respiratory tract, for example, produces mucus that traps encroachers. As for any microbes that we swallow in our food, they’ll likely meet a fiery death in the cauldron of the stomach, whose industrial-strength acid could literally burn a hole in your shoe. Should all these defenses be breached, immune cells will rush into the battle. This army is led by sentries that flag the intruder, and they’re followed by white cells that devour it and still...

With firepower like that you’d think humans would always be on the winning side. But parasites have huge advantages over us. Their population size dwarfs our own by staggering numbers, and their rapid replication rates ensure that there will always be a lucky few with mutations that will give them the upper hand. The battle between hosts and parasites is an unending arms race. In this intensely competitive environment, any parasites that by chance hit on ways to modify the behavior of a host so as to enhance their own transmission—perhaps, for example, by nudging it a wee bit closer to the parasites’ next host—would very swiftly multiply. Since hosts can’t evolve as quickly to thwart every new trick parasites deploy against them, their best chance for survival is to acquire traits that offer them broader protection. Mutations that prompt an animal to feel repelled by common sources of contagion—for instance, murky green water, a dung heap, or other members of its flock acting strangely—might serve that function.

The answer to that question is what set Janice Moore on her life’s path.

“It’s pretty funny when you think about it,” said Wright. “Caffeine is the most widely used drug in the world, and bees have been consuming it tens of millions of years before we showed up on the planet.”

But according to the standard medical wisdom, healthy people who were exposed to T. gondii typically developed only brief flulike symptoms, after which the parasite quietly nestled down inside their brain cells and caused no further health problems. It became, in medical parlance, a “dormant infection.”

And he learned that cleaning a cat’s litter box is far from the only route by which people become infected. You can get it by eating vegetables that haven’t been scrubbed properly or by failing to wash your hands after gardening. Grazing livestock can also pick up T. gondii from the ground, and the rapidly replicating organism not only travels to the animal’s brain but also produces thick-walled cysts in its muscle—the flesh that we eat. For that reason, people who consume undercooked beef or lamb are at greater risk of acquiring the infection.

Yet another way humans become exposed is by drinking water contaminated with cat feces—a common occurrence in the developing world, where a staggering 90 percent of people have the latent infection.

Flegr launched a study of 592 residents of central Prague and found that people with the parasite were 2.7 times more likely to be involved in traffic accidents than age-matched controls free of the infection. Since the strength of epidemiology lies in numbers, he conducted a follow-up study of a much larger population, tracking the incidence of traffic accidents in 3,890 Czech military draftees. Those identified as having the parasite at the start of the investigation went on to have significantly more crashes. “I estimate that as many as a million road deaths a year can be blamed on toxo,” Flegr told me.

What’s more, female rats are far more receptive to copulating with infected males. “It’s a very strong effect,” Ajai Vyas, Sapolsky’s postdoc at the time of the discovery, told me. “Seventy-five percent of the females would rather spend time with the infected male.” The parasite also invades the male’s sperm so when he copulates with a female, it can infect her pups, creating still more vehicles for transporting T. gondii back into the belly of a cat.

What made the subject so compelling for him boiled down to this: “Most of us are comfortable with the idea that a painkiller or drug might modify our behavior, but there’s something very different about a small parasite—a few hundred or thousand of these single-celled parasites—that’s in your brain for your entire life. And because you can’t get rid of them and you don’t know that they’re there, like at what point does all of their influence just become who you are?”

“Intellectually,” he said, reflecting back on the day, “I like the idea of there being caveats to free will, but I never thought about there being costs to caveats to free will or the practicalities of readjusting the whole infrastructure of society.”

Save for the ancient Egyptians, he found, virtually no one kept cats as pets

until the latter part of the 1700s. The first people to embrace the practice “were poets—avant-garde, left-wing types in Paris and London, and it just came to be the thing to do.” They called it the “cat craze,” and coinciding with it, the incidence of schizophrenia rose sharply.

“Many times we don’t know why we do what we do,” he said. “We usually associate mood disturbances with conflicts in early childhood, but who knows? Some of our unconscious may be controlled by pathogens.”

Fever is so important for slaying germs that animals that cannot regulate their own body temperature—for example, locusts, baby rabbits, and cold-blooded creatures like lizards—have found alternative means to cook pathogens: They sunbathe.

Vomiting is not just a means of getting rid of harmful microbes but also a preventive measure. A phenomenon known as sympathetic vomiting occurs when the sight of someone throwing up causes others to do so too. Such copycat behavior probably evolved to protect us from food poisoning, a hazard that was both more common and more lethal in generations past.

The addition of saliva to the petri dish prompted cells in the area of the injury to grow much more quickly than those not so treated. In the right situation, said Hart, “the old saying ‘go lick your wounds’ is very good advice.”

copulating, male rodents, cats, and dogs will furiously lick their penises for several minutes. The liberal application of saliva kills several pathogens that are leading causes of STDs in these species. Their habit also benefits females because it prevents males from passing on infections to their next mates.

Since food, especially meat, spoils more quickly in hot climates, Sherman and Billing predicted that the highest use of spices in the past would be in tropical regions—an idea they tested by combing through the ingredients of thousands of meat-based recipes from centuries-old cookbooks. True to their expectations, in warmer countries—notably Thailand, India, Greece, and Nigeria—all traditional dishes with meat used at least one spice and some twelve or more. Our ancestors from colder climates, in contrast, had much blander diets. For example, one-third of traditional meat-based recipes from Scandinavia called for no spices at all. A subsequent study of traditional recipes containing only vegetables—a less inviting growth medium for bacteria—found that they had fewer spices than those with meat, providing further support for their hypothesis.

Harvard psychologist Steven Pinker calls disgust “intuitive microbiology,” noting that “germs are transmissible by contact,” so “it is not a surprise that something that touches a yucky substance is itself forever yucky.”

The reason for the double standard is that you’re immune to your own

germs, and chances are you’ve already been exposed to the germs of an intimate other, so they’re not likely to hurt you either. Consequently, the filth and bodily emanations of those furthest removed from your social circle evoke the most revulsion.

Pizarro has a deep distrust of using disgust as a moral compass. If people rely on it, as they often do, it can lead them astray, he warns. In his class, he offers the denunciation of homosexuality on the grounds that it’s repugnant as a prime example of the dangers of disgust-based morality. “I tell my class: As a heterosexual male, it’s not as if I won’t be disgusted if you show me pictures of certain sexual acts between two males. The task for me is to say: What the hell does this have to do with my ethical beliefs? I tell them, the thought of two very ugly people having sex also revolts me, but that does not lead me to consider legislating against ugly people having sex.” The homeless are another group that people frequently speak ill of, probably because they, too, can trigger disgust alarms, making it easier for society to dehumanize them and find them guilty of crimes they didn’t commit.

In a study of 237 Dutch citizens published in 2014, those who scored highest on an online test of disgust sensitivity were more likely than their less squeamish countrymen to vote for the socially conservative Freedom Party (Partij voor de Vrijheid), which takes a strong position against immigration, is hostile toward Islam, emphasizes the value of Dutch traditions over a multicultural ethos, and has long favored exiting the European Union.

Psychopaths—whose ranks swell with remorseless cold-blooded killers—are notorious for their lack of empathy, and they typically have smaller than normal amygdalae and insulae, along with other areas involved in the processing of emotion. Psychopaths are also less bothered than most people by foul odors, feces, and bodily fluids, tolerating them—as one scientific article put it—“with equanimity.”

Interestingly, women rarely become psychopaths—the disorder affects ten males for every one female—and they have larger insulae than men relative to total brain size. This anatomical distinction may explain why they’re the sex most sensitive to disgust, and it may also have bearing on yet another traditionally feminine characteristic: as befits women’s role as primary caretakers, they score higher than men on tests of empathy—a very useful trait for gauging when a cranky baby has a fever or needs a nap.

To her, manners are what separated us from animals and allowed us to take “the first baby steps” en route to becoming civilized super-cooperators. Indeed, she thinks manners may have paved the way for “the great leap forward,” an explosion of creativity fifty thousand years ago manifested by specialized hunting tools, jewelry, cave paintings, and other innovations—the first signs that humans were sharing knowledge and skills and working together productively. Manners set our species on the track toward progress, but to truly become civilized, humans needed a more elaborate code of conduct, one that would bind communities together. They needed religion. Fortunately for humanity, it emerged just when it was most needed—namely, when our ancestors stopped wandering the world and decided to put down literal roots. About ten thousand years ago, some hunter-gatherers began experimenting with a radical new lifestyle: farming. Only a few of them did so at first, but the movement picked up steam, and gradually more people began to settle down, trading the wandering life for a patch of land, typically by a river delta.

The Torah contains much more medical wisdom—and by that I don’t mean merely its famous admonishments to avoid eating pork (a source of trichinosis, a parasitic disease caused by a roundworm) and shellfish (filter feeders that concentrate contaminants) and to circumcise sons (bacteria can collect under the foreskin flap, so removing it is believed to have lowered the spread of STDs). Jews were instructed to bathe on the Sabbath (every Saturday); cover their wells (a good idea, as it kept out vermin and insects); engage in cleansing rituals if exposed to bodily fluids like blood, feces, pus, and semen; quarantine people with leprosy and other skin diseases and, if infection persisted in the community, burn their clothes; bury the dead quickly before corpses decomposed; submerge dishes and eating utensils in boiling water after use; never consume the flesh of an animal that had died of natural causes (a sign that it might have been felled by illness) or eat meat more than two days old (likely on the verge of turning rancid).

When it came time for divvying up the spoils of war, Jewish doctrine required any metal booty that could withstand intense heat—objects made of gold, silver, bronze, or tin—to “be put through fire” (sterilized by high temperatures). What could not endure fire was to be washed with “purifying water”: a mixture of water, ash, and animal fat, basically an early recipe for soap.

Equally prescient from the standpoint of modern disease control, Mosaic Law has numerous injunctions specifically related to sex. Parents were admonished not to allow their daughters to become prostitutes, and premarital sex, adultery, male homosexuality, and bestiality were all discouraged, if not banned outright. Religion is an ideal enforcer of good public health, for many of the behaviors most relevant to disease propagation occur behind closed doors, outside of public v...

But Hinduism, which evolved more independently, is equally obsessed with bathing before prayer and concerned about contamination of the body and what parts of it should be allowed to touch other objects or people (the left hand, for example, is used strictly for bathroom functions, so it is a grave offense for a Hindu to offer food to someone with that hand).

That argument is even more convincing when you consider that large segments of populations were decimated by plague and pestilence over that very period. Natural selection would have strongly favored people who believed in God or who, at the very least, were conscientious in obeying religious doctrine that served to protect their health. Most important, it would have favored the survival of people with a punitive streak—that is, those prone to stiffly penalize anyone who broke society’s rules.

Our brains are also prone to viewing primary disgust elicitors like blood and semen as agents of evil. In many cultures, a woman who has been raped is treated like a sinner. She is tarnished, sullied, no longer virtuous or to be valued. No man will be with her because she has been corrupted by another man’s crime. The fact that women menstruate has further stoked the flames of misogyny, for this “bad blood” is often seen as God’s curse, proof of their inferior moral status. In many cultures, menstruating women are confined to separate quarters so as not to contaminate others. Orthodox Jews are prohibited from sitting on a chair that a menstruating woman has occupied. Hindus must bathe and change their clothes if they come in contact with women in this “impure” state. Even in more secular pockets of the world, many couples—both men and women—believe it is wrong to have sex when a woman has her period.

Even more troublesome, a study of people serving as mock jurors found that those highly prone to disgust were more inclined to judge ambiguous evidence as proof of criminal wrongdoing, to impose stiffer sentences, and to see the suspect as wicked. Compared to their less easily revolted counterparts, they were also more prone to harboring an exaggerated sense of the prevalence of crime in their own neighborhoods. A related study whose participants included law students, police cadets, and forensic experts similarly showed that disgust sensitivity correlated with a tendency to judge crime more severely and punish the perpetrators with longer sentences—and this association held up even for veteran forensic experts who were accustomed to seeing gruesome evidence.

The notion that a single phenomenon—our species’ psychological adaptation to parasite stress—can shape entire cultures may sound simplistic, but plenty of scientists are open to the theory.

Atheism, by contrast, flourishes where there are very low parasite loads.

Countries under severe parasite stress were more likely to be controlled by dictators; gender inequality was pronounced, and wealth tended to be concentrated in the hands of a small class of elites. In contrast, countries with the least amount of infectious disease had wealth more equitably distributed; their women were on a more equal footing with men, and individual rights were far more extensive. They were overwhelmingly democracies.

When I pressed Thornhill to point to a country that exemplifies his philosophy, he shot back, “The whole of the Western world, because gradually there’s been a reduction in infectious disease. In the 1920s, you get chlorinated water. By the 1930s you have food-handling and sanitation laws. These spread very rapidly specifically in the West, but not outside the West. Around the same time you got antibiotics. By 1945, you got fluorinated water and it spread very rapidly. That knocked out all the mouth infections. In 1945, you also got DDT, which knocked out all the insect vectors of disease. Malaria went down the drain and [so did] other human diseases [spread by insects]. “In the 1960s, we got the cultural revolution: civil rights, women’s rights, the sexual revolution. All that stuff came out of the cleanup of infectious diseases specifically in the West. Liberal parts of the world. Outside of the West none of these things have occurred.”