You are a vast and complex ecosystem, a bipedal rainforest that is so much more than the individual brainy primate you probably envision. There are so many creatures on you and inside of you at this moment that “you” are outnumbered ten-to-one. That’s right; your personal cells, those with your DNA, are but a small minority compared with all the symbionts, indifferent freeloaders, and potentially dangerous invaders you currently host. You should also be aware that parasites are the Earth’s dominant life form: They may outnumber all other life by as much as four to one.
Given the great volume and diversity of the small life with which we are in close and constant interaction, we should not be surprised to learn that some of these microbes are capable of influencing our brains and behavior.
It is even reasonable at this point to ask if some are impacting major historical events. Biologists know that parasites are a primary driving force in the evolution of life. If they can manage that, why couldn’t they be meddling in the affairs of human culture, too?
Let’s consider the case of one fascinating parasite, Toxoplasma gondii. Thanks to our long and close relationship with cats, Toxo may currently infect two to four billion people worldwide—at least a third of the global population. Researchers found that 84 percent of pregnant women in Paris tested positive for Toxo. Some societies in Europe have infection rates above 90 percent. I know what you are asking yourself: Are thousands of these microscopic creatures in your brain? The answer is “maybe."
People can pick up Toxo easily enough from exposure to cat feces and cat scratches, and by eating contaminated fruit, vegetables and undercooked meat. So, no, one doesn’t have to be the classic "cat lady" with a litter box in every room to end up playing host to Toxo. Virtually everyone is vulnerable—but don't panic. We have been coexisting closely with Toxo for thousands of years. Besides, there is as yet no medical treatment that can get it out of you once it’s in.
So relax and keep reading.
This single-celled parasite prefers to be in a cat, so when it finds itself in a rat it manipulates the host in pursuit of its own desires. For example, Toxo will slow the rat’s reaction time, making it easier for a cat to catch and eat the rat—a good outcome for Toxo. It can also make rats lose their innate fear of cats, including even making a rat sexually attracted to the smell of cat urine. There's more: Toxo seems to tinker with human behavior in similar ways. Maybe it assumes humans are just big rats—not so far off base as one might assume given the relatively close ancestry we share, and the resulting similarities of mammalian physiology.
Multiple studies have indicated that Toxo seems to make those infected more reckless and less fearful than they would be otherwise. The question I would ask: What percentage of X-Games athletes and professional bull riders are infected with Toxo compared with accountants and stamp collectors? It also appears that it may influence infected men to be more jealous and suspicious of romantic partners. At the same time, other research suggests that Toxo may cause infected women to become more social and trusting of strangers. For women in environments that make them vulnerable to physical violence and sexual assault, this could be reckless behavior.
Researchers discovered that drivers and pedestrians in Prague who were involved in traffic accidents were more than twice as likely to be hosting Toxo in their brains than other residents of the city. This would seem to point directly to the Toxo playbook of slowed reaction time and increased reckless behavior.
Toxo also has been implicated in some mental-health conditions. For example, people with this parasite are more likely than others to suffer from depression and anxiety. Some scientists are now willing to say out loud that they suspect Toxo is behind as many as one-third of all cases of schizophrenia. I would not be surprised if we see a radical shift in our thinking about mental illness in the coming years. What have been traditionally thought of as genetic and/or trauma-induced problems may turn out to be, in many if not most cases, the result of parasitic invasions.
Let’s think loose and free for a moment: Could Toxo be a major player in world history? Don’t dismiss this right away: Cats were domesticated perhaps more than 10,000 years ago, so it stands to reason that Toxo has been playing its games with human brains since at least then, and probably much longer, given our shared prehistoric environments with various cat species. How many of history’s daring heroes, villains, and revolutionary figures might have been nudged or spurred on to some degree by these microscopic interlopers? Some? Most? The idea that one microscopic species most people have never heard of could be so important to us individually and collectively is humbling.
The strange case of Toxo leads one to wonder—and worry—about how many other microbes, not yet identified or understood, are influencing human moods and behaviors. It is possible that we are all operating with programming written to a high degree by gatecrashers. A study published this year in the journal Nature Communications found that changes in gut bacteria influence brain activity, including behavior associated with depression and anxiety. Mounting evidence seems to implicate certain viruses as being behind several mental disorders.
If parasites play a profoundly influential role throughout the natural world—and they do—why should we imagine that we are somehow above being their pawns and playthings? All our concrete, steel, and plastic may provide the illusion of separation and safety from nature’s monsters and manipulators, but don’t be fooled: Medical science has made us safer, but we are still vulnerable.
Perhaps the greater lesson here is that we are not set apart from the rest of nature as so many mistakenly believe. No: We are nature; nature is us. Moreover, our dance with other life forms is far more intimate than previous generations ever knew or imagined, even as they lived deep within the natural world. The reality that science continues to reveal is that there is a near-seamless flow of life, from the invisible microbes within us to the large plants and animals we see and touch. Knowing and accepting this may be a key step in our intellectual evolution. We are not so much individual human beings, but unique networks, vast collections of many life forms and many lives, flowing through time together.
Guy P. Harrison is the author of six non-fiction books that promote science and reason. His new book is Good Thinking: What you need to know to be smarter, safer, wealthier, and wiser.
Some information for this essay was drawn from the following recommended sources: