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.
via io9.com
If this was in a sci-fi movie, you’d call it silly.