Except this might not be the case. Oxford scientists have recently conducted a series of studies that suggest toxoplasma has a subtle but noticable change on the brain. For example, uninfected rats avoid areas with the smell of cat urine for obvious defense reasons. However, rats infected with toxoplasma not only showed no ill reaction to the smell of cat urine, but even sought it out.

The question now becomes: does toxoplasma have any effect on humans? This article explores the possible link between toxoplasma (which can be found in about half the human population) and schizophrenia. Examples are also given of other parasites that have devastating effects on animals, such as this one:

"Hairworms, which live inside grasshoppers, sabotage the grasshopper's central nervous system, forcing them to jump into pools of water, drowning themselves. Hairworms then swim away from their hapless hosts to continue their life cycle."

-LiveScience.com: Mind Control by Parasites