To the best of our knowledge, insidious parasites like these only exist in science fiction. However, there is an organism on Earth -- a single-celled protozoan to be specific -- that bears a striking resemblance. And alarmingly, it may infest as much as a third of humanity (including 45% of the French)!
That parasite is Toxoplasma gondii. Commonly found in domestic cats, it sexually reproduces only within feline intestines. Humans can contract the parasite by ingesting anything contaminated with cat feces. Once infected, individuals may succumb to mild, flu-like symptoms for a couple weeks, but little more. Often no symptoms manifest at all. The parasite's apparent harmless nature cloaks its presence. Soon after finding its way into a human host, it weasels its way into the brain. There, "sheltered from the full fury of the immune system," it can remain in what's called a tissue cyst for decades, not reproducing, just persisting. But in this form, researchers are finding that it can produce some subtle and startling changes in the host's behavior.
For example, biologist Jaroslav Flegr, one of the leading researchers on T. gondii has discerned some very peculiar findings when comparing people with the infection to those without it. Kathleen Mcauliffe reported on this last year in The Atlantic:
Additionally, people infected with T. gondii display slightly impaired motor skills, undertake more risks, and get into more automotive accidents. Furthermore, 38 studies have linked the parasite with schizophrenia, a fact which fascinates neurologist and Yale assistant professor Steven Novella.
...males who had the parasite were more introverted, suspicious, oblivious to other people's opinions of them, and inclined to disregard rules. Infected women, on the other hand, presented in exactly the opposite way: they were more outgoing, trusting, image-conscious, and rule-abiding than uninfected women.
"Some drugs which are typically used to treat schizophrenia actually have anti-toxo effects," Novella said on a recent episode of The Skeptic's Guide to the Universe. "So they may work because they're actually counteracting toxoplasma, itself."
How T. gondii subtly reprograms the brain is still not well understood. Studying the parasite outside of the body, researchers have found that it profoundly affects gene expression in host cells, as well as molecules involved in signal transduction pathways. If neurons in the brain are influenced in the same manner, that could possibly give rise to the behavior-altering effects.
Worldwide, as many as 1 in 3 people have T. gondii in their brains. Scary, I know. But hold off on any urges to cull the cat population. Indoor cats pose little to no threat to their owners, and the prevalence of infection is only about 1 in 10 in the United States (and currently declining). However, that's still a significant amount, a preponderance that makes you wonder how much of an effect these microscopic, mind-altering parasites really have on human interactions and everyday life as we know it.