its a desease which is carried by cats in their brains and can only reproduce sexually in cats, but can reproduce asexually in other mammels. it alters rats brains in such a way that they actively seek out cats so the cat can catch it and hopefully get the disease. my theory is that in humans it creates the cat love that we see so much :-) (quite a lot of the population is infected but in most it doesn't have any serious affect)