Such ups and downs can alter an entire ecosystem. If a rancher is overgrazing his sheep on a semiarid grassland, the sheep may multiply and the plants will dwindle. At the same time the grazing changes the parasites: with more sheep available, they can breed in huge numbers, and they crowd on the dwindling blades of grass, making the probability that a sheep will become infected even higher. In other words, overgrazing automatically triggers an outbreak and scales back the herd, allowing the grass to recover. Soon the sheep population bounces back as well, but thanks to the management of the parasites, it never gets large enough to turn the grassland into desert. Rather than loading up their livestock with antiparasite drugs, and thereby ruining their grazing lands, ranchers may benefit by letting parasites keep the herd in check.
For now, though, the theory of parasitic stability remains mostly theory because scientists know so little about parasites in nature—which is another reason why Daniel Brooks is in Costa Rica. “People will be able to test their ideas on parasite stability because this won’t be a parking lot in thirty years. Parasites may dampen oscillations, and if they are having an influence, you don’t want to eradicate parasites.”
To manage Guanacaste, in other words, you need to understand its parasites. “If we want to preserve a place like this,” Brooks said, “we have to know what’s going on microscopically. We need to figure out how to work with parasites. We need to figure out what organisms need and want, so we can use them in ways that don’t terminate their existence.”
The way Brooks was talking about us humans reminded me of the way parasites use their hosts—evolving a sense of what their hosts need and want, what they can and can’t live without—so that they don’t destroy themselves. In my travels for this book I often thought about the natural world as the sum of its parts. I would look down out of planes at the mud lakes of Sudan, the circuit-board housing tracts around Los Angeles, the disintegrating ranches and scraps of forest of Costa Rica and think about a concept, called Gaia, which some scientists embrace. They think of the biosphere—the rind of ocean, land, and air that’s home to life—as a kind of superorganism. It has a metabolism of its own, which shuttles carbon and nitrogen and other elements around the world. The phosphorus that helps power the flash of a firefly ends up in the soil when the firefly dies, perhaps to be taken up by a tree and added to one of its leaves, dropping into a river and flowing to the sea, where photosynthesizing plankton take it up, only to be eaten by some grazing krill, which releases it into the ocean depths in its feces, only to be taken up by some bacterial scrounger, and cycled back up to the ocean’s surface, before finally, many years later, ending up entombed in the sea floor. Like our own bodies, Gaia is held together and kept stable by its metabolism.
We humans exist within Gaia, and we depend on it for our survival. These days we live by using it up. We strip topsoil away with our farms without replacing it; we fish out the seas; we clear out forests. I thought about what Brooks had just said, about learning how to use nature without terminating it.
“You talk as if we were a parasite,” I said.
Brooks shrugged his shoulders. The idea was fine with him. “A parasite that has no self-regulation is going to put itself out of existence and may take its host with it,” he said. “And the fact that most species on Earth are parasites tells us that hasn’t happened a lot.”
I chewed that over for a while. Here was a new meaning parasites could have for us—one that could take the place of Lankester’s degenerates, Jewish tapeworms, and all the old myths of failed evolution. One that could be biologically faithful without turning life into a horror movie, without having parasites come bursting out of our ribs. It is we who are the parasites, and Earth the host. The metaphor may not be perfect, but it chimes well. We reroute the physiology of life to our own ends, mining fertilizer and blanketing farm fields with it, much as the wasp reroutes the physiology of its caterpillar to make the kind of foods it needs. We use up those resources and leave behind our waste, like Plasmodium turning a red blood cell into a garbage dump. If Gaia had an immune system, it might be disease and famine, which can keep an exploding species from taking over the world. But we have dodged these safeguards with medicines and clean toilets and other inventions, and they’ve allowed us to put billions of people on the planet.
There’s no shame in being a parasite. We join a venerable guild that has been on this planet since its infancy and has become the most successful form of life on the planet. But we are clumsy in the parasitic way of life. Parasites can alter their hosts with great precision and change them for particular purposes: to take them back to their ancestral home in a stream, to move on to their adulthood inside a tern. But they are expert at causing only the harm that’s necessary, because evolution has taught them that pointless harm will ultimately harm themselves. If we want to succeed as parasites, we need to learn from the masters.
Epilogue
While I was writing Parasite Rex I went on a string of blind dates. A friend of mine had decided to become my matchmaker, having heard the bit of Jewish lore that three successful matches gives you automatic entry into heaven. The fact that my friend was a Chinese Muslim did not diminish her zeal. Unfortunately, by the time she was done with me, she was no closer to a home in the clouds. The dates fizzled for all the various reasons that dates fizzle. However, one in particular still sticks in my memory today, over a decade later. I was sitting with a woman in a restaurant terrace on a warm night in Greenwich Village. Surrounded by paper lanterns, we were discussing what we did for a living. She told me about advertising. I said I was writing an entire book about how amazing parasites are. She tried to change the subject of the conversation. It was as if I had stuck a thorn in the evening’s bicycle tire. I could almost hear the gentle hiss as it slowly and steadily went flat.
As I described the book on that ill-fated night, I realized what a strange, isolated world I had entered. I regularly drew out life cycles of parasites, marking up paper napkins with arrows traveling from snails to ants to birds. I knew which species of blood fluke infects the blood vessels behind your intestines and which one dwells behind your bladder. I thought that Louis Pasteur should move over and make room in the history of science for Friedrich Kuchenmeister, the tapeworm pioneer, even though I suspected that I was the only person in my time zone who even knew who Kuchenmeister was.
Fortunately, by the time Parasite Rex was published in 2000 I was happily engaged to my wife, Grace, who was not scared off by my obsession. And once people had a chance to read the book, I discovered many kindred spirits. A radio producer asked me to appear on her show, saying that I had given her nightmares for a week. She meant it as a compliment. At a party at the New York Public Library, a high school librarian introduced herself to me. She told me that Parasite Rex had been stolen six times from her library, setting a record. I took that as a compliment as well. The very least I could do, the librarian told me, was to talk to her students. A few weeks later I arrived at her school, carrying a carousel filled with the goriest selection of slides I could find.
Sometimes when I traveled to talk about parasites I would meet people who had their own stories to tell me. On a visit to Johns Hopkins in 2006 a malaria expert told me about a strange sight he saw one day in Zambia. As he walked down a road, he saw a wasp and a cockroach in front of him. When he came up close to them, it looked as if the wasp was leading the cockroach by pulling an antenna, like a dog on a leash.