Science writer Carl Zimmer's new book, "Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures," introduces readers to some of nature's most sinister characters: nematodes that cause blindness, worms that swell up a scrotum until it fills a wheelbarrow, 60-foot-long tapeworms and deadly creatures so tiny they hitchhike on the back of a fly.

Zimmer, who lives in New York and is a contributing editor at Discover Magazine, says that—parasitically speaking—there is an embarrassment of riches all around us.

But it wasn't pure science that first piqued his fascination (dare we say obsession?) with this world of bloodsuckers. As a kid, he always got a kick out of parasite-type science fiction movies. So, in the name of research, he not only traveled to far-off places but fired up the VCR and watched a gruesome marathon of jelly-like aliens, hairless bipeds and gut-devouring lizards vying—literally—for the heart of man.

The result? He knows far more about parasites than even he bargained for. Salon talked with Zimmer by phone about malaria, monster movies and the possibility of parasitic world domination.

Q: "Parasite" is such a loaded, metaphoric word. We think of welfare mothers, ex-wives, dinner guests who don't return the invitation, Hitler's label for the Jews. Technically though, what is a parasite?

A: Anything that thrives at the expense of what it's living on and living in. In the broadest definition, you include viruses, a lot of bacteria and things you don't normally think of as parasites. A fetus in the womb actually behaves a lot like a parasite. It uses strategies to extract nutrients and energy out of its mother. And the mother, to a certain extent, has to defend herself against it.

In the scientific world, parasites have been cast as minor hitchhikers, not a serious force of nature. Yet, you say that parasites make the world go 'round.

Historically, they were viewed as agents of disease. Or they were seen as nature's degenerates—animals and organisms that had devolved and lost all ability to live in the free world. Scientists thought you could look at the world and ignore the parasites.

But in fact, they are everywhere. Open up any animal—healthy or sick—and it's just loaded. No one really knows how many parasites exist, but there are estimates of four species of parasites for every nonparasite. The vast majority of species are parasites. They may, in fact, have a very powerful evolutionary effect.

Q: Does that mean that parasites drive everything from eating to mating behavior?

A: Sex is a good example. Why did sex evolve? If you look at it, it really doesn't make much sense compared to just cloning yourself. So scientists consider factors that might make sex an advantage. One of the best theories is that parasites make sex desirable because they are always trying to adapt to their hosts, take advantage of and in some cases, kill them. Anything an organism can do to defend itself is going to give a good evolutionary advantage. The way that sex shuffles up genes can help give a species an edge.

It depends on the person and where in the world you go. In the United States, you might open up a person and maybe in their brain you would find cysts of a single-celled parasite called toxoplasma. Somewhere between 30 to 50 percent of the U.S. population carry it in their brains. In France, the proportion goes up to 90 percent or more.

Toxoplasma normally lives inside cats and they shed it out in their feces and then it moves to its next host, which is a rat or a bird or some other prey of a cat. Once the parasite gets into the intermediate host, it replicates and then builds a protective shell around itself. Each of these cysts contains a few hundred toxoplasma. The cysts can sit for years. The parasite is basically waiting for its host to be eaten by another cat so that it can begin its life cycle again. People pick up toxoplasma all the time, whether they are gardening or handling kitty litter. It likes to be in the brain.

Q: Cysts in the brain? Wouldn't we notice that?

A: The cysts will just sit there, waiting for us to be eaten by a cat. (Toxoplasma is usually harmless in humans, except for pregnant women and people with compromised immune systems.)

But there is some really interesting research. When toxoplasma cysts are in a rat, the parasite somehow does things to the host that makes it more likely to be eaten by a cat. For example, it takes away the rat's healthy sense of fear. Rats with toxoplasma are less likely to run away in terror at the smell of a cat.

Q: Does that apply to humans? Once infected, do we get an urge to sign up for a safari and put ourselves at the mercy of big game?

A: Some psychologists have actually done studies comparing people who have toxoplasma and people who don't. They found very subtle personality differences. People who have toxoplasma may be more openhearted than people who don't.

Q: So a parasite could be responsible for altering the personalities of billions of people?

A: Hookworms are a definite example of that. Up until the mid-20th century, they were rampant in the United States, especially the South. They penetrate your skin, burrow into your body and live in the intestines. The whole reputation of the lazy Southerner comes from the fact that a lot of them had hookworms. They weren't lazy. They were anemic. Up until recently, Americans had lots of parasites.

Q: Confession time. I've had my share of parasites. Giardia, head lice and, OK, pinworms. How about you?

A: I'm basically parasite free. I've traveled a lot in the developing world. So far—knock on wood—nothing serious. If it's any consolation, pinworms are everywhere, and in everything.

If you have to have something, that's not so bad compared to what's out there in the rest of the world. There's a fluke called schistosome that lives in the bloodstream. It infects 200 million to 300 million people a year in Africa and southern Asia. There are 1.2 billion cases of hookworm in the world. Two or 3 million die of malaria a year. And many of these parasites are on the rise, not the wane.

Q: What parasite ranks highest on your personal yuck scale?

A: A little crustacean that lives in the ocean. It swims inside the mouth of certain fish and devours their tongue and plants itself down in their mouth and proceeds to act like their tongue. Quite disconcerting.

Q: Is there a pinup parasite?

A: Actually, when you look at them awhile, some of them are downright pretty. For instance, tapeworms have a little head that is adapted for letting them hold on inside their host's intestine. Some heads look like ram's horns. Some look like arrows. Some look like dandelions. They are all different and quite beautiful.

Q: Give us a mini-biography, from birth to death, of one of the more sinister parasites.

A: The Guinea worm starts off its life cycle living inside a microscopic copepod that swims around in fresh water. When a person drinks the water, the stomach acid dissolves away the copepod. But the larva manages to survive and burrows its way out of the intestinal wall. It wanders around the abdominal cavity looking for a mate. The male Guinea worms get up to 2 inches long. The females can get up to 2 feet long.

When they mate, the male dies. The female starts traveling through the connective tissue of a person's body, down into their leg. All the while, its fertilized eggs are developing. It crawls to the leg and starts releasing the larvae just under the skin. That creates a very painful blister that people generally want to wash off in water.

Which is exactly what the parasite wants the person to do! The blister pops and the larvae go into the water. When the Guinea worm senses the water being poured on it, it will slowly start pushing itself out of a person's body and releasing more larvae. Once in the water, they go off looking for another copepod.

Q: Assure me there's a cure.

A: There's no drug you can take if you get Guinea worms. The only "cure" is one that's been around for thousands of years. As the Guinea worm is pushing itself out of your body, you slowly wrap it around on a stick. You don't want to grab it and pull it out because it will break and die, and then you have a 2-foot-long parasite in your body that will cause infection and might very well kill you.

For thousands of years, people have lain around for days just turning these sticks. Some think that this procedure is the symbol of medicine—the snakes around the staff. There are references to fiery serpents in the Bible when the Israelites are wandering in the desert. Guinea worms. Fortunately, even though there's no vaccine, they may be eradicated soon through public health awareness on how to avoid them [for example, using cheesecloth to drink fresh water and learning to not wash in freshwater sources].

Q: Good riddance to Guinea worms—and pinworms and head lice, too. But, don't we have to consider the big picture? If we are living in a parasitic world and they are such a dominant force, are we messing things up by eradicating them?

A: Truthfully, total eradication is just a dream. They are so resourceful and abundant. But in terms of medicine, there may be some unexpected results when you get rid of parasites.

Scientists are looking into the possibility that eradication might be responsible for allergies. There's a pretty clear correlation. Places where parasites, such as intestinal worms, have been eradicated are the places where you see the most allergies and disorders like Crohn's disease (an autoimmune disease in the intestines).

In Venezuela, scientists found that affluent city dwellers had high rates of allergies and very few parasites. Poor people living in the cities, because their sanitation isn't as good, have more parasites but their allergy rates are consistently lower. Same with Indians dwelling in the rain forest.

Right now, these are tantalizing connections. You have to keep in mind that our ancestors have been grappling with parasites for millions and millions of years. In a sense, we have an uneasy truce with them. Part of that involves the way the immune system holds them in check. Taking away parasites all of a sudden may make the immune system prone to overreacting to things like cat dander, peanuts or even your own body.

Q: Attacked by parasites and attacked by our own bodies. Sounds like a standard-issue horror story scenario. In fact, some of our deepest, most universal fears are based on parasite images. “Alien.” “Invasion of the Body Snatchers.” Is there any possibility of some kind of catastrophic invasion?

A: There are certainly lots of parasites that we don't know about out there, whether they are virus, bacteria, worm or fungi. There are lots of things in the natural world that make their living by invading other organisms.

A lot of them are living inside hosts that live in the rain forest and remote parts of the world. The more we disturb these environments and the more globalized human society becomes, the easier it is for something to hop out of its previous host and try to infect humans. It may find that humans are a perfectly good host.

Q: If we hold up the mirror, what can we learn about ourselves from parasites?

A: If you think about the relationship we have with the rest of nature, the way we extract from the natural world, the way we use up resources without restoring them, the way that we manipulate nature in order to make it serve us better: These are all things that parasites do, and do very well.

After learning so much about them, I don't think that it is necessarily a bad thing to be called a parasite. They are quite clearly the most successful organisms on the planet.

But it might be useful for us to get an idea of what they are doing right. Because if we humans are parasites, I don't think we are very good ones. A parasite that kills off its hosts has got nowhere to live. Human beings only have one host. We have to treat it accordingly.

Copyright © Salon, September 2000