NEW YORK -- Perhaps you've never encountered a Yeerk -- a slimy, slug-like creature, gray-green and the size of a rat. But maybe you have.
You just didn't know it. Because maybe the Yeerk crawled into the skull of your best friend or your brother, and the human is now a slave to the Yeerks, and to their fiendish plot to take over the Earth.
"We're toast," as Marco would say.
Marco is an Animorph. And if you don't know about Animorphs, the next Big Thing in the world of children's literature, an obsession to a million young readers -- well, heaven protect you from the Yeerks.
So far, there have been 18 Animorphs books and offshoots. They feature Jake, Rachel, Marco, Cassie and Tobias, five kids who happen into the last moments of a dying warrior from another world.
The Andalite warrior reveals the Yeerk plot and bestows upon the children the ability to "morph" into other species.
They then go about the business of saving the world when they're not doing their homework or cruising the mall.
Perhaps it is the cool business of characters changing into animals and then struggling to control the animals' instincts. Perhaps it is the idea of normal kids performing heroic acts.
For whatever reason, young readers are forming Animorphs clubs. They are posting scores of Animorphs Web pages, writing their own stories and listing dozens of clues about obsession. ("You're afraid to get up in the middle of the night to go to the bathroom because you're afraid you'll set off alarms and the Yeerks will catch you.")
They are bombarding author K.A. Applegate with more than 100 e-mail messages every day, appending countless exclamation points.
"I LOVE THESE BOOKS!!!!! It's sick . . .," writes Andrew. "I keep them in a safe place with a lock so no one can get to them and where they will stay forever and ever because these books are AWESOME!!!!!!!!!!"
Writes another fan: "I love Animorphs!!! You are the best author!!! . . . The books are the most interesting!! If anyone asks what do I want the most, I would say more Animorphs!!! Keep working hard, remember your fans love you!! P.S. We'll protect you from the Yeerk filth!!"
There are other, less fanatic fan letters -- from parents and teachers, and even the occasional college student who appreciates the quality of Katherine Applegate's fiction. Not long ago, she was surprised to hear from a 40-year-old fan.
"I told him, `You need to get a hobby,' " says Ms. Applegate, 41.
The author of more than 100 books, ranging from pop-up books to adult romances, Ms. Applegate lives quietly in Minneapolis with her 9-month-old son, Jake. That he shares his name with an Animorph is just a coincidence, she says. She avoids most interviews and has never met her editors.
Animorphs installments arrive by mail at her publishers, Scholastic Inc., at an amazing clip of one 180-page book a month. "She will not be persuaded to come to New York," says Jean Feiwel, editor-in-chief. "I've tried to lure her with all kinds of interesting things. She just won't come."
The intensity of her readers and their fierce connection with her characters is "like having thousands and thousands of editors," Ms. Applegate says. "I feel so obligated to deliver for them."
Her primary audience is the 8-to-12-year-old reader. It is estimated that half are girls -- unexpected converts to science fiction. They may be drawn to the strong female characters: Rachel, perhaps the most courageous of the Animorphs, and Cassie, the books' moral center.
Each book is narrated by a character. Jake is the leader; his brother, Tom, is a "Controller," enslaved by a Yeerk. Marco is the group's wisecracker, a (seemingly) motherless boy who is drawn only reluctantly into the Animorphs' heroics:
"Excuse me," Marco said, "but does anyone else ever stop to realize that some of the things we talk about doing are totally insane?"
"What? Turning into fish, so we can be carried by a hawk and let ourselves be sucked up into the pipe of an alien spaceship, so that we can then turn into tigers and gorillas and whatever, and overpower the creepy aliens?" Rachel said. "Is that what you mean by insane?"
"That's it exactly."
"Yep," Rachel said, "it is insane."
And then there is Tobias -- abandoned by his parents, bullied by his classmates. In the course of one battle, Tobias morphs into a red-tailed hawk and remains a hawk too long. He cannot change back into a boy.
In a way, Tobias' plight is typical; the world of Animorphs, for all of its wisecracks, is a dark place where bad things happen to good people.
"I don't share that Hollywood mentality that every ending has to be a happy one," Ms. Applegate says. "Kids are capable of understanding the gray areas, and they want to explore that."
Their parents and teachers, meanwhile, "appreciate that my stories deal with responsibility and freedom and pacifism -- the kinds of things you might not encounter in other books."
The popularity of Animorphs, with 10 million paperbacks sold at $3.99 each, has been a tonic for Scholastic, which has seen its Goosebumps books decline.
Ms. Feiwel says there will be Animorphs T-shirts and electronic games. And Animorphs, already popular in France and England, will soon be published in Indonesian, Hebrew, Italian, Norwegian, Dutch and Spanish.
Come the fall, there will be an Animorph television series on the Nickelodeon cable network. You might think that this news would be received with unalloyed glee among the books' fans.
You would be wrong.
"I know that many of you are with me in my feelings that the Animorphs television show is a terrible idea because it will ruin our mental images," writes Kim Smith, in her Club Morph newsletter on the Web.
Though Ms. Feiwel certainly hopes the television show will be a success, "it's very gratifying for a publisher to see that the book reigns."
Ms. Applegate, too, has mixed emotions. She has been assured that the program will follow her stories, at least at first, but she is afraid they will be "a little manhandled. . . . I really hope that Marco still has that sense of humor and Rachel is still gung-ho."
She is under contract to write 30 Animorphs books and says she intends to keep writing them until she is bored. Strangely, she says, an ending to the serial "has never crossed my mind."
Could Animorphs end with the defeat of our young heroes and the Earth in the thrall of the Yeerks?
"Hmmm," she says. "I don't think it would be right to end this whole world, to end the entire series unhappily."
Author K.A. Applegate turns into a lion in this computer-enhanced promotional photo for her series of Animorph books. In the books for children, some characters "morph" into animals such as lizards, wolves and elephants.