Well, toxoplasma gondii. Toxoplasmosis is the disease it causes. It infects and estimated third of the world's population but is generally considered harmless. I suppose that would be "harmless" in the same sense as asbestos was once regarded as harmless. This is an incredibly devious protozoan if I might be permitted to anthropomorphize for a moment. It can only reproduce in cats so it's primary objective is to infect an animal that will be eaten by cats. The preferred vector seems to be mice. Once it infects a mouse, it dulls the animal's sense of fear and actually causes it to have an affinity for cat urine - thus making it even more likely that it will be caught and eaten. However there is some evidence that it also has similar effects in people. article edit: the article mainly focuses on it's link to brain cancer, but since there was a great article in SciAm Mind that focused on the behavioral aspects (potentially) and since I can't post links to that article, I decided to quote this one for that purpose.