Toxoplasma gondii might be the most famous brain-controlling parasite of all. It's a single-celled organism that can infect almost any warm-blooded animal, although it only reproduces sexually in cats. In rats, infection by T. gondii has been shown to affect their behaviour – making them attracted to the smell of cat urine and fur, and less afraid of new things in their environment, two behaviours which may make them more likely to be killed by cats. (The parasite can also be transmitted between rats sexually – and there is evidence that infected male rats are more sexually attractive to female ones.)
But where it gets unsettling is the possibility that the parasite, which is hugely common in humans, affects our behaviour too. "There are lots of studies finding correlations between [human] infection and behaviour," says Weinersmith. "There was also a study that found a correlation between country-wide neuroticism scores and the percentage of the population infected by Toxo, suggesting that the parasite influences culture."
One researcher, Jaroslav Flagr – himself a carrier of the parasite – found when testing Czech students (40% of Czechs are carriers), that people infected by T. gondii had slower reactions than the uninfected. And, interestingly, men who were infected were more likely to be introverted and unsocial, while women who were infected were more outgoing and gregarious. There's a great long piece on Flagr's research into human T. gondii infection in The Atlantic.
Weinersmith thinks it's likely that Toxo really does manipulate human behavior, but says it will be much harder work to prove it conclusively. "We'll never be able to conduct the kinds of experiments that will really nail down the effects of Toxoplasma gondii on human behavior," she says. "That would require infecting a random subset of folks with the parasite, and then quantifying how their behavior changes. That pesky ethics thing makes this kind of experiment impossible." The best we can do is follow people who have been infected with it and compare them to people who haven't, but that makes it harder to tease out cause and correlation. After all, people who own cats are probably more likely to be infected by a cat parasite. "If people who own cats have different behaviours than non-cat owners, this could mess up our interpretation of the results," says Weinersmith.
However, there is at least one other ironclad example of a parasite which controls human behaviour: rabies. Humans and other mammals infected by the rabies virus develop hydrophobia – a fear of water – and aggressive behaviour, including biting. Since the virus is spread by saliva, that makes sense: Biting for obvious reasons, while water could wash infected saliva away and make the bites less infectious, so a fear of water is good for the virus. Clearly, humans are not beyond the reach of behaviour-controlling parasites.