Cats are the main host for Toxoplasma which can only reproduce in a feline's body. To spread its species in absence of legs or wings, Toxoplasma requires a way of getting more cats to ingest its offspring. That is where the rat comes in. Toxoplasma spreads by infecting hosts which are eaten by cats.
The Toxoplasma organism lives benignly in the brain of rats and other animals, using its host for just enough energy to survive, generally without harming the host. It is in its best interest for an infected rat to remain healthy, that is until it encounters a cat. Then it ends its friendly relationship, encouraging the rat host to get eaten. This is where the zombie comes in.
The new study by Patrick House of Stanford University suggests that the Toxoplasma infected brain causes the rat to lose some of its fear of a cat while maintaining its normal anxiety and fear brain responses to other predators. The male rats brain also shows stimulation response to rat urine odor in an area usually activated when a female rat approaches. The result is similar to the fate of a turkey gobbler responding to a hunter's call.
"Simply living within another organism - locating it, traveling through it, finding food and a mate inside, altering the cells that surround it, outwitting its defenses - is a tremendous evolutionary accomplishment. But parasites such as Sacculina do more: they control their hosts, becoming in effect their new brain and turning them into new creatures. It is as if the host itself is a simply puppet, and the parasite is the hand inside."You will be startled by the number of parasites that are around and even in us, and their influence on behavior. Then try to get a good night's sleep.
More on Toxoplasmosis and suicide at Scientific American.