The single-celled parasite Toxoplasma gondii lives infects rats, but it needs to be inside a cat's digestive system in order to reproduce. The parasite actually alters the brain of its rat host so that it won't be afraid of cats.
Specifically, Stanford researchers discovered that Toxoplasma affects the rat's brain so that the fear centers of the brain no longer respond to cat odors. Even more crazily, it appears that Toxoplasma makes the rat brain think it's sexually attracted to the cat odor. Those factors are likely more than enough to get rats hanging around dangerously close to cats, and thus gives the parasite a chance to complete its reproductive cycle.
The parasites appear to be very precise in their alterations - the rats still function normally in all areas not directly related to the fear of cats. Researcher Patrick House explains:
"These findings support the idea that in the rat, Toxoplasma is shifting the emotional salience of the detection of the cat. They also suggest that fear and attraction might lie on the same spectrum, or at least that the emotional processing of fear and attraction are not entirely unrelated."
We don't know how the parasite has this remarkable effect. Previous research indicates that Toxoplasma tends to enter the rat's brain and take up residence near the amygdala, a part of the brain heavily involved in fear and other emotional responses. Somehow, Toxoplasma is causing certain subsections of the amygdala to decrease the fear response to cat odor.
And it might not be only rats who are affected by this. A third of all humans carry Toxoplasma, and we don't really have a clear grasp on what - if anything - these parasites might do to the human brain. There's some evidence that Toxoplasma is linked to incidents of schizophrenia in humans, but what we don't know still far outweighs what we do.