The new science of evolutionary medicine asks the question: who (or WHAT) benefits when people show symptoms of a disease? Often, it's the microbes that are causing the disease in the first place. For instance, cold symptoms. When an infected person coughs or sneezes out some of the tiny organisms that are causing the problem, they are able to "colonize" and infect someone else, and thus propagate themselves.
Almost every multicellular animal is home to these kinds of fellow travelers, each of which has its own agenda, which in some influence--or even take of--part or all of the body which they are inhabiting. In effect, they are controlling their hosts' behavior.
Many fly and wasp species lay their eggs inside hosts, such as bees. As the larvae feeds on its host, it alters its behavior in a way that favors the continued survival and reproduction of the parasite.
A type of tapeworm causes its mouse host to become obese and sluggish, making it easy prey for predators like a fox, meaning the tapeworm can move into the next phase of its life cycle.
One kind of fluke starts its life cycle inside a snail, then moves on to an ant. The ant manipulates the ant's brain so that it will climb to the top of a blade of grass and wait there until it is eaten by a grazing sheep. Once inside the sheep, the worm releases its eggs, which then exit through the sheep's feces. Which are then eaten by snails, and the cycle starts all over again.
In the October 7th of the New York Times, evolutionary biologist David P. Harash writes: "And this, in turn, leads to the question: who's in charge of your own mind? Where does the rest of the world end, and each of us begin?
"Think of the morgue scene in the movie 'Men in Black,' when a human corpse is revealed to be a robot, its skull inhabited by a little green man from outer space. Science fiction, but less bizarre than you might expect, or want to believe." (NOTE: Subscribers can still listen to these shows).