This thread keeps getting creepier. A reader writes:
There is good evidence that Toxoplasma gondii -- the parasite often found in cat feces that poses a risk to human fetuses -- spreads by affecting the behavior of its rodent hosts. Infected rodents show decreased fear-responses to cats, which is thought to increase the likelihood that the cats will then eat the rodents, allowing the protozoa to complete the next phase of their reproductive cycle in a feline host.
Your reader wrote, "Can you image something like that in humans? Scary." There is a growing body of evidence, which is somewhat controversial, that toxoplasmosis may induce behavioral alterations in infected humans as well.
Specifically, increases in risk-taking behaviors appear to be correlated with infection. Wikipedia has a good summary of the evidence, and relevant caveats. Carl Zimmer's book "Parasite Rex" is a great introduction to the weird and wonderful realm of parasites, and their incredible adaptations and ability to co-opt their host's biology.
On his blog, Zimmer reviews recent research that suggests the microbes in our gut may influence what we eat (and how big we get): "Mice with a genetic make-up that alters the diversity of their gut microbes get hungry, and that hunger makes them eat more. They get obese and suffer lots of other symptoms. Get rid of that particular set of microbes, and the mice lose their hunger and start to recover."
Another reader sends an APA article that examines a link between T. gondi and schizophrenia. Another writes:
Yet another reason to chose dogs over cats!