That's the gist of an article in this month's Atlantic, profiling the work of biologist Jaroslav Flegr:
[I]f Flegr is right, the “latent” [Toxoplasma gondii] parasite may be quietly tweaking the connections between our neurons, changing our response to frightening situations, our trust in others, how outgoing we are, and even our preference for certain scents. And that’s not all. He also believes that the organism contributes to car crashes, suicides, and mental disorders such as schizophrenia. When you add up all the different ways it can harm us, says Flegr, “Toxoplasma might even kill as many people as malaria, or at least a million people a year.”
Flegr was especially surprised to learn, though, that the protozoan appeared to cause many sex-specific changes in personality. Compared with uninfected men, 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.
The idea that parasites affect our behavior is relatively new, but gaining ground. And T. gondii may not be the only one that affects human behavior. This is a bit of science to watch.