Cal is a freshman studying biology in New York City, a fresh transplant from Texas, when he wanders into the wrong sort of bar and is picked up by the wrong sort of girl. Of course, at the time Morgan seemed like exactly the right sort of girl, being that he was drunk, and that she was ready, willing, and able to help him lose his virginity.

Cal's relationship with Morgan turns out to be nothing more than a one-night stand, so he tries to forget about her—which isn't too difficult considering how drunk he was, and that he never even learned her last name. That his first sexual experience wasn't something memorable isn't terribly unusual, but what is strange is that his first and only real girlfriend, Sarah—along with every other girl he ever kisses after his encounter with Morgan—turns into a raving, bloodthirsty maniac.

See, Morgan is what is known as a parasite-positive, or a "peep," and peeps have this nasty habit of passing along their parasites via sexual contact. But the disease Cal contracts is nothing so prosaic as HIV or herpes; instead, he's infected with the parasite responsible for what legends refer to as vampires.

But Cal is one of those rare people who have a partial inborn immunity to the parasite, and although he's infected with vampirism, he's what's known as a carrier—a vampire Typhoid Mary. He carries the parasite, but is immune to its effects. Or mostly immune; he doesn't fear sunlight or thirst for blood (though he does often crave a nice piece of rare meat), but he does enjoy some of the perks, like a hypersense of smell, night vision, and increased strength.

After being infected, Cal is recruited by an ages-old shadow organization known as the Night Watch, a group that predates the formation of the United States, as do some of its carrier peep members. The Night Watch's mandate is to keep the peeps under control, and to protect normal humans from this hidden menace.

So things could be worse. Cal finds himself with an interesting, if extremely dangerous, career, some cool superpowers, and a really long life span to look forward to. But after hunting down, capturing, and rehabilitating his girlfriend and the other girls he'd kissed, he still hasn't come to terms with his disease, and feels a sense of disquiet. And so he's given a task: he must find his progenitor—the elusive Morgan—if he is ever to learn to cope with being a peep.

That won't be as easy as it sounds, considering all he has to go on are hazy recollections, a first name, and the memory of a drink called a Bahamalama-Dingdong. This, in a city of millions, isn't much help. But when a handbill advertising the drink is brought to Cal's attention, he has somewhere to start his investigation . . . an investigation that will take him into the deepest, darkest parts of the New York City underground, where he will discover the roots of a conspiracy, the true nature of his affliction, and how to live in this new world.

Westerfeld's prose is pellucid—tight and clean, with hooky chapter endings that defy the reader to put the book down. The first-person narrator tells the tale in a believable and engaging young-adult voice, which is full of droll asides and keen observations of the world around him; likewise, the dialogue is both witty and true. So too are the characters well drawn and empathetic; Cal, the protagonist, and Lacey, his unwitting partner in his investigation, are a vibrant, vividly realized hero/heroine duo. As their biologically forbidden desire for each other unfolds, anyone who has ever has had his or her love denied cannot help but identify with their struggle.

But perhaps what's most compelling about Peeps is the sense of verisimilitude Westerfeld brings to the vampire archetype. His rationales for the various myths of vampire lore are both delightful and logical. Parasite-positives are not at all the standard vampires of legend. Everything from a vampire's bloodlust to their renowned aversion to sunlight is explained scientifically. For example, a peep's loathing of crosses and sunlight is due to a side effect of the disease known as the "anathema effect," which makes peeps grow to hate what they once loved. Hundreds of years ago when vampire myths first arose, most Europeans were Christian, so this is what the vampiric cruciphobia is derived from. Interesting tidbits like this, along with some fascinating parallels drawn between the fictional vampirism parasite and real parasites, make for a fascinating read.

With Peeps, Westerfeld has crafted an infectious and clever reinvention of the vampire novel: one that turns the archetype on its head, and makes what is old new again. Though Peeps is marketed as YA, adults should have no compunctions about picking up this, or any of Westerfeld's other YA novels. He is a natural storyteller, and like Lloyd Alexander or J. K. Rowling, his fiction can—and should—be enjoyed by all ages.

John Joseph Adams is the assistant editor at The Magazine of Fantasy & Science Fiction. He reviews science fiction for Kirkus and audiobooks for Publishers Weekly. His nonfiction has also appeared in Amazing Stories, The Internet Review of Science Fiction, Locus Magazine, Locus Online, and Science Fiction Weekly. For more information, visit his website, or his blog, The Slush God Speaketh.