Do Parasites Rule the World?

In a TED talk last week, British science journalist Ed Yong told us about a caterpillar that spent the last hours of its existence as a "headbanging zombie bodyguard defending the offspring of the creature that killed it"—and showed us how fairly common this sort of thing is, in the natural world:

http://www.youtube.com/watch?v=CfqO1U6lfDs

The question he asks near the end is whether we might also be controlled, in less dramatic ways, by parasites. As Yong points out, there have been studies on possible subtle effect the fairly ubiquitous parasite toxoplasma gondii might be having on its human hosts. But toxoplasmosis is fairly unique, because it's already a fairly well-understood infection that has more dramatic effects on other mammals. What about the trillions of other bacterial cells in and on our bodies—which collectively weigh more than the human brain, and outnumber our naturally-occurring cells 10:1? Might some of them have a hidden agenda?

The Human Microbiome Project will help us answer some of these questions by cataloging the bacteria that we can find inside of our bodies, but—in terms of scale—doing microbiology in the entire human body is very much like doing zoology on the entire planet Earth. With apologies to Walt Whitman, we contain multitudes—each of us a secret universe of microbial flora, every microscopic creature fighting for its survival.