Yup. Looking at parasitism, I was reminded about brood parasitism: http://en.wikipedia.org/wiki/Brood_parasite What about a little exsurgent strain that make infected people care for something awful and alien, thinking about it as if it was their child - with all the intense love and bonding people feel for a baby. They are otherwise entirely normal, but will do anything to help the cuckoo... "Stop complaining Brian! If your little sister wants to eat your leg, you should let her! You are so spoiled."