"Whence, thinkest thou, kings and parasites arose?" (Percy Bysshe Shelley, "Queen Mab").
"Parasite Rex" is an interesting look at the evolution and biology of parasites--but only for the strong of stomach.
In a sense it was comforting to read that we are not the most savage, efficient predators on Earth. Witness the parasite Sacculina that turns crabs into zombies--the slug-like female enters the crab through a convenient leg joint and gradually fills its whole body cavity, even wrapping roots around its host's eyestalks. The crab continues its shadow-crab life, sidling through the surf, eating mussels and clams. However, it stops moulting and growing as these activities would funnel energy away from Sacculina. If the parasite becomes pregnant, it doesn't matter whether the crab started out as male or female--it will brood and hatch the next generation of Sacculina as if the tiny larvae were its own children.
Sacculina's life-style is rather hard on the crab, but it is only one of the amazingly efficient (and bizarre) parasites described in this book. Other parasites have adopted Sacculina's method of eliminating their host's unnecessary functions such as reproduction (unnecessary to the parasite, that is) while leaving the host enough brain, nervous system, and digestive tract to go on feeding.
I imagine if there were a Sacculina-type parasite for humans, we would soon lose our urge to play football or go swimming. We would lie in front of the TV all day long, stuffing ourselves with---wait a minute, here! Is it possible...?
Zimmer also describes the stratagems that potential hosts have evolved to eliminate, or at least confuse the parasites that view them as mobile homes with well-stocked refrigerators:
"Consider the leaf-rolling caterpillars. They're pretty ordinary insect larvae with one exception: they fire their droppings like howitzers...What on Earth could have driven the evolution of an anal cannon? Parasites could. When parasitic wasps home in on a larva such as the leaf-roller caterpillar, one of the best clues is the odor of their host's droppings...The intense pressure put on leaf-roller caterpillars by [parasitic] wasps has pushed the evolution of high-pressure fecal firing. By getting their droppings away from them, the caterpillars have a better chance of not being found by wasps."
Even plants have evolved defenses against parasites, usually by creating poisonous chemicals that the parasite ingests as it chomps on the plant. Some plants will even emit a scent that attracts predator wasps to the caterpillar that is munching on their leaves (so much for high-pressure fecal cannons).
Be sure to read Chapters seven and eight: "The Two-Legged Host; and "How to Live in a Parasitic World," and ponder the statement, "A parasite that has no self-regulation is going to put itself out of existence and may take its host with it."
Could we humans be considered parasitic to Earth, itself?
"Parasite Rex" has detailed footnotes, a "Further Reading and Selected Bibliography" section, and an index for those readers who would like to pursue this fascinating subject. Carl Zimmer has also written, "At Water's Edge" and is a frequent contributor to "Discover," "National Geographic," "Natural History," "Nature," and "Science."