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.

"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?

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.

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

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.