Alien Invasion


Nothing makes more marketable copy than lurid descriptions of some fiend tearing apart an innocent human body. Now that lions and tigers are on their way to extinction, it is the microscopic fiends—especially parasitic ones—which present the best options for gruesome infotainment. First off the mark with this new genre of real-life horror was Richard Preston’s bestseller The Hot Zone. Interspersed with scientific passages on the Ebola virus were dripping accounts of Ebola-infested organs turning to mush and leaking out of every conceivable orifice. Think Stephen Jay Gould meets Stephen King.

Two new books follow in The Hot Zone‘s footsteps, though both are considerably more sober. In Parasite Rex, science writer Carl Zimmer takes the broad approach, giving us an introduction to the vast range of human parasites that plague our globe. Arno Karlen, a New York psychoanalyst and independent scholar of biomedical history, trains his sights in Biography of a Germ on a single representative of the parasitic classes, the bacterium responsible for Lyme disease.

Microscopic parasites have always been far greater threats to human life and limb than lions, sharks, or killer bees. The malaria parasite alone kills 2 million to 3 million people a year, and HIV has devastated much of southern Africa. Parasites also bring the horror of mutilation. Filarial worms, the parasites responsible for elephantiasis, can make a scrotum swell to the size of a wheelbarrow, while Onchocerca volvulus, a coiled worm as long as a snake and thin as a thread, provokes a rash so profoundly itchy that people scratch themselves to death. If the worms get into the eyes, blindness ensues, and in some places in Africa they have claimed the sight of almost everyone over 40.

Zimmer tells us cheerily that “most people on earth carry parasites, even if you set aside bacteria and viruses.” Over 1.4 billion people carry the snakelike roundworm Ascaris lumbricoides in their intestines; almost 1.3 billion carry bloodsucking hookworms. Scientists now know that parasites make up the majority of species on earth—according to one estimate they outnumber free-living species four to one. “In other words,” Zimmer writes, “the study of life is, for the most part, parasitology.”

Yet the study of parasites is a relatively new science. Until the late 19th century, most scientists believed that parasitic creatures were not separate species but manifestations of moral or physical malaise spontaneously generated within the body—symptoms rather than causes of disease. Louis Pasteur and germ theory put them squarely on the map. Darwin’s theory of evolution showed moreover that parasites were part of the great tree of life, and hence our distant relations. More recently biologists have suggested that parasitism plays a key role in evolution; Lynn Margulis has proposed that eukaryotic cells (of which all plants and animals are composed) originated when ancient cells were parasitized.

But it is parasites’ badass image that makes them such a hot topic. Take the ichneumon wasp, whose eggs hatch in the guts of caterpillars, where the larvae paralyze their hapless host and slowly eat it alive, careful to preserve essential organs till the last possible moment. Darwin himself saw the ichneumon as an antidote to sentimental ideas about God—how could a benign Creator be responsible for something so heinous?

A good deal of Parasite Rex is devoted to detailing the extraordinary machinery that parasites employ in their ingenious survival strategies. We humans may be in the middle of a biotech revolution, but compared to most parasites we are rank amateurs. Consider the humble liver fluke: Even without a brain it can navigate through a human or animal body, unerringly making its way to the liver. In tropical countries flukes kill countless cattle and cause $2 billion in damage a year, yet it took scientists decades to understand how this mindless creature finds its way around.

Perhaps my favorite example is the parasitic barnacle Sacculina, whose larva injects itself into a crab and slowly fills the animal’s entire body, its roots even wrapping round the crustacean’s eyestalks. “It is as if,” Zimmer writes, “the host itself is simply a puppet, and the parasite is the hand inside.”

All this is marvelous “Yuck!”-inducing stuff; the problem is that the beasties begin to blur together in the mind. Those who loved Ridley Scott’s Alien are sure to get a kick, but just as one episode of Alien is better than three, so a few chapters of Parasite Rex will probably be enough for most readers. Something similar might be said about Biography of a Germ, nearly 180 pages devoted to the parasite responsible for Lyme disease.

Again, one cannot but be impressed by the ingenuity of the Borellia burgdorferi (Bb) spirochete. With its dwarfish genome of just 1283 genes (small even as bacteria go), it must battle the defenses of creatures, from ticks to human, that are vastly more complicated than itself. “Yet none is more resourceful nor ultimately more successful,” Karlen writes admiringly. Here then is an almost loving portrait, a genuine biography, complete with in-depth “character” analysis and “family” history.

But Karlen does have a critical message that goes beyond the specifics of the Lyme parasite. The reason Lyme disease burst onto the scene was human intervention in the environment. Ironically, it was our desire to return cultivated land to wilderness that created much of the problem. Second-growth forests are ideal breeding grounds both for ticks, who carry the Bb spirochetes, and for the deer and mice who serve as their primary targets. Around the world all sorts of parasites, many of them far more ruthless than Bb, are on the rise because of human disturbance of local ecosystems.

Both Zimmer and Karlen make clear that parasites are not out to get humans; for many of them we are irrelevant or inadvertent hosts. But with their truly alien chemical arsenals they pose a dire threat to human life. Moral of the story: We encounter them at our own peril.

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