“It is definitely not smart to intentionally infect yourself. I’ve already had people ask.”
A third of the world has been infected, though. Tiny cysts nested in one's brain and muscles attest. The parasite Toxoplasmosis gondii comes into us by undercooked meat, well-intentioned placentas, gardening soil, or, most infamously, cats. It is the reason that pregnant women are not supposed to empty litter boxes.
“If you’re young and healthy and have it already, it might provide some benefit, as we saw in our research,” Ann-Kathrin Stock, a cognitive neurophysiology researcher at the University of Dresden in Germany, told me. “But the adverse effects are potentially huge. If you ever really get sick it might be what kills you.”
Many people have what feels like a cold after they get infected with Toxo. The symptoms pass, and the person feels fine. But the Toxo lives on inside them, hidden dormant in little cysts, kept in check by constant pressure from the person's immune system. If our immune systems become weak, because of a serious illness later in life, though, the Toxo can break out and attack organs like the brain or retina.
“You might lose your ability to see, or lose your cognitive faculties,” Stock said.
Neuroscientist Joraslav Flegr, an eminent voice in Toxo research, told The Atlantic last year that, “Toxoplasma might even kill as many people as malaria, or at least a million people a year.”
What does it mean to learn that it can also have beneficial effects?
Toxo has been all over the news in recent years, since it became known that the parasite manipulates people’s behavior. Maybe most interestingly and notoriously, it seems to make men more introverted, suspicious, unattractive to women, and oblivious to the way others see them. Infected women, inversely, have been shown to be more outgoing, trusting, sexually adventurous, attractive to men, and image-conscious. Infected men tend to break more rules than their uninfected peers, and infected women tend to pay them more heed. Infected men and women are 2.5 times more likely to have traffic accidents, more likely to develop schizophrenia, and more likely to engage in self-directed violence.
As these stories made news, though, they lacked a logical explanation. At least 60 million Americans have it, if almost always unknown to them, so understanding this is beyond academic. Thinking about the potential scale of the parasite’s effect on civilization and history can be overwhelming, like imagining the twin brother you never had. If you do have a twin brother in real life, it’s like imagining your triplet. Already have a triplet? You get the idea.
Why, though? Why does Toxo affect human behavior? In The New York Times last year, Choire Sicha channeled cats in a memo (“From the desk of: Cats”) that read, “We have trained, by means of this gentle biological warfare, your women to let us into your homes, and your men to stay home and scratch us in our difficult places.”
Who are you going to believe? The cats?
Toxo manipulates its hosts indiscriminately, according to the most prevalent hypothesis. The parasite just wants to improve its odds of disseminating, living on. These fascinating human behavior changes are serendipitous, equal-opportunity mind control.
“It interferes with brain chemistry, but the parasite itself doesn’t ‘intend’ to harm someone,” Stock told me. “It always acts by the same mechanism, whether it’s in humans or rodents. It’s just that humans are very rarely prey to cats. That doesn’t help its goal. We’re a dead-end host.”
That means its effects in humans are various. Some might be good, and some are bad, but none are intended to incapacitate us. We are collateral damage.
Stock is the lead author on a study published this week in the journal Brain, Behavior, and Immunity that found an apparent contradiction in the traditional understanding that the parasitic infection is essentially negative for humans.
“We had people sit in front of a monitor,” Stock explains, “and they were required to press a button with one hand if a ‘go’ signal appeared. When a ‘stop’ signal instead appeared, they were required to shift and press a button with their other hand. In this part of the experiment, the people who were infected were consistently, significantly faster to respond.” When the task was simpler, all ‘go’ and no ‘stop,’ the infected subjects performed the same as the uninfected.
They describe this as an improvement in action control; the first study to demonstrate this. It is somewhat at odds with research by Flegr that demonstrated delayed reaction times in infected people (a leading explanation for the traffic accidents). Flegr’s experiments involved different tasks, though, which might involve different, brain pathways.