This Is Your Brain On Parasites: How Tiny Creatures Manipulate Our Behavior and Shape Society

by Kathleen McAuliffe

Parasites and hosts have been competing with each other for billions of years. The first bacteria were parasitized by the first viruses. When larger, multicellular life forms emerged, these microbes in turn colonized them. Meanwhile, parasites continued evolving into a menagerie of distinct forms—roundworms, slugs, mites, leeches, lice, and the like. As life grew in size and complexity, natural selection favored parasites that were the best at evading hosts’ defenses, and hosts with the greatest skill in repelling the invaders. Today, virtually every aspect of the human body’s design bears witness to this age-old struggle. Our most visible defense is our skin, which provides a thick barrier to the hordes of microbes that populate its surface. Entry points are especially fiercely guarded: Eyes are bathed in tears that flush out intruders. Ears are lined with hairs to keep out bugs. The nose has a filtration system for screening pathogens out of the air. Invaders that make further inroads will only encounter stiffer resistance. The respiratory tract, for example, produces mucus that traps encroachers. As for any microbes that we swallow in our food, they’ll likely meet a fiery death in the cauldron of the stomach, whose industrial-strength acid could literally burn a hole in your shoe. Should all these defenses be breached, immune cells will rush into the battle. This army is led by sentries that flag the intruder, and they’re followed by white cells that devour it and still...