But Aeby has found that the alliances in her ecosystem are different from the ones that Lafferty has uncovered in salt marshes. When a killifish brings a fluke to a bird, the killifish dies in the process. But corals consist of colonies of clones and when an individual polyp infested with a fluke dies, it is replaced by a healthy new one. An infected polyp can’t feed or reproduce, so allowing a fluke to fester inside it is a drain on the colony, slowing its growth. If a butterfly fish prunes the coral, it can perform as well as a healthy coral. It’s to the coral’s advantage to get rid of its sick polyps, which may mean that the coral is actually contributing to the color or spikes in order to make it easier for the butterfly fish to spot. Lafferty found a case in which a parasite and its final bird host were allied; here, Aeby has found a case where the intermediate host and the parasite work together.
Discovering parasites at work in ecosystems can feel a bit like watching in terror as a bank robbery unfolds and then looking across the street and seeing a movie crew with its cameras and boom mikes. Birds are being guided to their meals, and fish are choosing their coral polyps, thanks to the advertisement of flukes. Uncovering these effects is hard work, and only a few examples have been documented. But they’re enough to suggest that parasites can cast some of the hoariest notions of ecology into doubt. We tend to think of predators as keeping a herd of prey healthy by weeding out the slowest ones. That’s not what’s happening in Lafferty’s salt marsh, or even among those icons of predator and prey, the wolf and the moose.
Wolves are the final hosts for one of the smallest tapeworms in the world, Echinococcus granulosus. Far from a ticker-tape ribbon, it’s lucky if it gets to be a quarter of an inch long as an adult. It doesn’t cause its final host much harm, but its eggs can be vastly vicious. They are eaten by herbivores such as moose, where they slowly transform themselves into cysts in which thirty individuals may sit. They will keep growing if there’s no bone in their way. When they accidentally end up in humans, they have been known to grow so big that they’ve contained fifteen quarts of fluid and millions of baby tapeworms.
One of the tapeworm’s favorite sites for forming its cyst is the lungs. A moose may carry several in its lungs, each tearing through its bronchial tubes and blood vessels. As a result, when wolves sweep down on a herd of moose, they’re more likely to pick out the slow, wheezing one and kill it. It’s even possible that these moose tapeworms can create the same kind of scent used by rat tapeworms to lure beetles. Instead of leaving the scent in droppings, though, the moose tapeworms could release their aroma with their host’s every breath. In any case, the result is that the tapeworm brings the wolf to the moose so that it can get into the wolf. The thinning of the herd is an illusion, not the service of the predator but the side effect of a tapeworm traveling through its life.
On my way to see Lafferty, I stopped one night in a hotel in Riverside, California. It had originally been a Spanish mission, and after unpacking, I prowled around the old shrines, explored the hidden passageways surrounded by vines and palms, crossed the hushed stone courtyard. I came back to my room feeling utterly alone. I turned on the television for company. An episode of The X-Files was on. As well as I could figure out, an FBI man had suddenly turned gloomy and wouldn’t return anyone’s phone calls. When another agent tracked him down and confronted him, the gloomy man threw him to the floor and brought his face close to his, opening his mouth. With wonderful creaking and slithering noises, a scorpionish creature crawled out of his throat and climbed into the other agent’s mouth.
I didn’t feel so lonely after that. Some television screenwriter had parasites on his mind as well. It occurred to me that parasites were the basis for a lot of science fiction novels, of movies and television shows. And I was struck by the fact that these parasites were dangerous because they could manipulate their hosts, just as parasites can in reality. When I got back home I started renting videos. I told my friends, and they’d tell me about other movies I should see, books to read. It got to be a gruesome marathon. The oldest entry I could find was Robert Heinlein’s The Puppetmasters, a 1955 novel. A spaceship full of aliens travels from Saturn’s moon Titan and lands near Kansas City. But the aliens inside aren’t the standard-issue 1950s hairless bipeds; they’re pulsating jellyfish-like creatures that latch onto people’s spines. Hiding underneath the clothes of their hosts, they tap into their brains and force them to help spread the parasites across the planet. The fight against them is a bit ludicrous, with the government forcing everyone to walk around practically naked to be sure they’re not carrying an alien. Humanity is saved when the army finally finds a virus that can kill the parasites, and the book closes with a fleet of spaceships leaving Earth for Titan to exterminate the parasites for good. It’s a stiff, peculiar book—the only one I’ve read that ends with the battle cry “Death and Destruction!”
The Puppetmasters was turned into a pretty mediocre movie in 1994, but its essence—the notion of humans harboring giant parasites—has become a Hollywood institution. Parasites are a part of our shared dramatic language, just as they were in Greek comedies. Any blockbuster can rest its plot on parasites without anyone’s worrying that it will seem too esoteric. One of the biggest movies of 1998, The Faculty, takes place in a high school where parasites from another planet are taking over the bodies and minds of teachers and students. These fluke-like things sprout tentacles and tendrils, and they pull themselves into their new hosts through their mouths or ears. Their hosts change from frazzled teachers and sulking, violent kids to glazed-eyed upstanding citizens who try to spread the parasite to new hosts. It’s up to the assorted losers of the school—drug dealers, geeks, and dropouts—to save the world from the invasion.
Parasites got their first big break at the movies almost twenty years earlier, in the 1979 movie Alien. A spaceship hauling ore stops off to investigate a crash on a lifeless planet. The crew discovers an alien ship that has been destroyed in a ruthless attack, and nearby they come across a clutch of eggs. One of the crew, a man named Kane, takes a close look at one of the eggs, and a giant crablike thing bursts out of it, clamping to his face and wrapping a tail around his neck. His crewmates bring him back to their ship, alive but comatose. When the ship’s doctor tries to get the thing off him, it tightens its tail around Kane’s neck. The next day it has disappeared, and Kane seems fine. He gets up and eats voraciously, to all appearances normal. Of course, no movie monster ever just disappears. This one has been devouring Kane’s guts, and before long he suddenly clutches his stomach, writhing and screaming, and a little knobby-headed alien pierces through his skin and leaps out. As the parasitic wasp is to the caterpillar, so this alien is to humans.
Alien may have made Hollywood safe for parasites, but a lot of the conceptual legwork had already been accomplished four years earlier in a low-budget, little-seen movie directed by David Cronenberg called Shivers. It is set on Starlight Island, an immaculate high-rise building on an island outside Montreal. “Sail through life in quiet and comfort,” says the soothing voice-over on a commercial for the building. But the isolated quiet and comfort is destroyed by an engineered parasite. It’s the work of one Dr. Hobbs. Dr. Hobbs originally set out to create parasites that could play the role of organ transplants. A parasite could be connected to a person’s circulatory system and filter blood like a kidney, for example, while taking only a little blood to keep itself alive. But Dr. Hobbs also has a secret agenda: he’s decided that man is an animal that thinks too much, and he wants to turn the world into one giant orgy. To that end he fashions a creature that will be a combined aphrodisiac and venereal disease: a parasite that will make its hosts sexually voracious and will be spread during sex.