The parasite Toxoplasma gondii is common in rats, but can only sexually reproduce inside the belly of a feline. The solution? Brainwash the rats into craving the scent of cat urine. Once infected by Toxoplasma gondii, rats who would normally have a phobia of cat urine actually seek it out, increasing their chances of getting eaten and transmitting the parasite into a feline gut.
“There are a million examples of parasites manipulating host behavior,” said Robert Sapolsky, a Stanford University neuroscientist. Just see the related article “Suicide Grasshoppers Brainwashed by Parasite Worms”.