Tons of cat owners have Toxoplasma gondii, the common feline parasite. We've known for quite some time that T. gondii makes rats lose their fear of cats, making them more likely to be eaten. We've also known that the parasite causes humans to become more reckless, and in some cases hornier.

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.