Think that's bad? Check out Toxoplasma gondii [http://en.wikipedia.org/wiki/Toxoplasma_gondii]. It reproduces sexually in cats. When it infects a rat or mouse, it overrides their instinctive fear of cats, making them get themselves eaten so that the protozoa can infect the cat that does so.
A protozoa that can change the self-preservation instincts of an animal it infects.
And one-third of the human population of the Earth is believed to be infected with Toxoplasma gondii. Acute infections are suspected of causing encephalitis and schizophrenia. Better hope it doesn't decide that it doesn't look at the rest of your brain and decide "That's some tasty lookin' prime-cut meat right there".
