Life Lessons for All of Us From the Eerie World of Parasites

Two hundred soldiers guard a besieged fortress where 1,000 children lie helpless. If the soldiers repel all invaders, the children will be able to reach adulthood and leave. But the soldiers themselves are effectively chained to their posts. They must die so that the children will live. If these selfless heroes were people, we’d bury them at Arlington.

But they aren’t people. They are larvae of a parasitic wasp, Copidosoma floridanum, which lays its eggs in the bodies of caterpillars of the cabbage looper moth. The eggs take a month--an eternity, in wasp terms--to mature. To protect the majority from rival parasites, a few grow up faster and assume guard duty. These “soldiers” are sterile and can never leave the riddled, dying body of their host.

Or consider the California killifish. It eats horn snails, which are infected with the parasitic fluke Euhaplorchis californiensis. The fish are eaten by shorebirds, which are infected in turn and leave droppings full of fluke eggs, which are eaten by the snails.

Researchers at a salt marsh in Carpinteria found that the flukes help this cycle along by flooding the fish’s brains with chemicals that alter their behavior. The fish lose their caution, swim close to the surface, flash provocatively. They are 30 times more likely than non-infected fish to be caught by birds. If the birds, whose own infection is relatively benign, had a vote, they’d probably approve the trade-off.

Science journalist Carl Zimmer’s survey of the growing field of parasitology is packed with such anecdotes. He’s careful to emphasize what complex and interesting creatures these are, rather than just their horror-movie aspects. Parasites, from bacteria to 60-foot tapeworms, not only make up three-quarters of the life forms on Earth, Zimmer argues, but are a primary agent of evolutionary change.

The human immune system has evolved in all its subtlety to fend off parasites, but, as AIDS has shown, parasites have developed counter-weapons. How does a tapeworm, a hookworm, a virus “hide” in a body trying to destroy it? How does a liver fluke unerringly navigate through the body to find the liver? Scientists, Zimmer says, have begun to find the answers.

Zimmer, the author of “At the Water’s Edge,” a contributing editor at Discover magazine and a columnist for Natural History, emulates his subjects. How can he get his message noticed and accepted by new hosts (us readers)? By altering our brain chemistry with writing that’s a model of liveliness and clarity.

“Parasite Rex,” indeed, is a book capable of changing how we see the world. After Darwin, Zimmer says, Victorian evolutionists viewed parasites as lazy, degenerate life forms. Later, health workers thought of parasites chiefly in terms of the diseases they caused. For the general public, they were nasty but peripheral.

Today, however, researchers are finding that parasites are such a huge part of nature that their absence is a sign of a polluted ecosystem. This may be true even at an individual level, Zimmer says: Affluent Westerners have rid themselves of intestinal worms only to become susceptible to allergies rare among Third World peoples--the immune system, without its accustomed enemies, sometimes goes haywire.

From a sleeping-sickness ward in Sudan to a forest reserve in Costa Rica, Zimmer looks over the shoulders of parasitology’s pioneers. His portraits of them, though vivid, pale next to his descriptions of the creatures being studied. We should emulate them, Zimmer says. They limit the harm they do so that their host won’t die--or at least won’t die too soon. We are parasites on the Earth--clumsy, destructive parasites. “If we want to succeed we need to learn from the masters.”