Welcome to the wonderful world of parasites.

This is where I live.

One more thing, and then I promise no more horrific biology (for a few pages).

When I first read about trematodes, I always wondered why the bird would eat this twitching, oddly colored snail. Eventually, wouldn’t the birds evolve to avoid any snail with a glowing left eye? This is a nasty, trematode-infected snail, after all. Why would you eat it?

Turns out the trematodes don’t do anything unpleasant to their flying host. They’re polite guests, living quietly in the bird’s gut, not messing with its food or its left eye or anything. The bird hardly knows they’re there, just craps them out into the next pond over, like a little parasite bomb.

It’s almost like the bird and the trematodes have a deal between them. You give us a ride in your stomach, and we’ll arrange some half-blind snails for you to eat.

Isn’t cooperation a beautiful thing?

Unless, of course, you happen to be the snail…

Okay, let’s clear up some myths about vampires.

First of all, you won’t see me using the V-word much. In the Night Watch, we prefer the term parasite-positives, or peeps, for short.

The main thing to remember is that there’s no magic involved. No flying. Humans don’t have hollow bones or wings—the disease doesn’t change that. No transforming into bats or rats either. It’s impossible to turn into something much smaller than yourself—where would the extra mass go?

On the other hand, I can see how people in centuries past got confused. Hordes of rats, and sometimes bats, accompany peeps. They get infected from feasting on peep leftovers. Rodents make good “reservoirs,” which means they’re like storage containers for the disease. Rats give the parasite a place to hide in case the peep gets hunted down.

Infected rats are devoted to their peeps, tracking them by smell. The rat brood also serves as a handy food source for the peep when there aren’t humans around to hunt. (Icky, I know. But that’s nature for you.)

Back to the myths:

Parasite-positives do appear in mirrors. I mean, get real: How would the mirror know what was behind the peep?

But this legend also has a basis in fact. As the parasite takes control, peeps begin to despise the sight of their own reflections. They smash all their mirrors. But if they’re so beautiful, why do they hate their own faces?

Well, it’s all about the anathema.

The most famous example of disease mind control is rabies. When a dog becomes rabid, it has an uncontrollable urge to bite anything that moves: squirrels, other dogs, you. This is how rabies reproduces; biting spreads the virus from host to host.

A long time ago, the parasite was probably like rabies. When people got infected, they had an overpowering urge to bite other humans. So they bit them. Success!

But eventually human beings got organized in ways that dogs and squirrels can’t. We invented posses and lynch mobs, made up laws, and appointed law enforcers. As a result, the biting maniacs among us tend to have fairly short careers. The only peeps who survived were the ones who ran away and hid, sneaking back at night to feed their mania.

The parasite followed this survival strategy to the extreme. It evolved over the generations to transform the minds of its victims, finding a chemical switch among the pathways of the human brain. When that switch is thrown, we despise everything we once loved. Peeps cower when confronted with their old obsessions, despise their loved ones, and flee from any signifier of home.

Love is easy to switch to hatred, it turns out. The term for this is the anathema effect.

The anathema effect forced peeps from their medieval villages and out into the wild, where they were safe from lynch mobs. And it spread the disease geographically. Peeps moved to the next valley over, then the next country, pushed farther and farther by their hatred of everything familiar.

As cities grew, with more police and bigger lynch mobs, peeps had to adopt new strategies to stay hidden. They learned to love the night and see in the dark, until the sun itself became anathema to them.

But come on: They don’t burst into flame in daylight. They just really, really hate it.

The anathema also created some familiar vampire legends. If you grew up in Europe in the Middle Ages, chances were you were a Christian. You went to church at least twice a week, prayed three times a day, and had a crucifix hanging in every room. You made the sign of the cross every time you ate food or wished for good luck. So it’s not surprising that most peeps back then had major cruciphobia—they could actually be repelled by the sight of a cross, just like in the movies.

In the Middle Ages, the crucifix was the big anathema: Elvis and Manhattan and your boyfriend all rolled into one.

Things were so much simpler back then.

These days, we hunters have to do our homework before we go after a peep. What were their favorite foods? What music did they like? What movie stars did they have crushes on? Sure, we still find a few cases of cruciphobia, especially down in the Bible Belt, but you’re much more likely to stop peeps with an iPod full of their favorite tunes. (With certain geeky peeps, I’ve heard, the Apple logo alone does the trick.)

That’s why new peep hunters like me start with people they used to know, so we don’t have to guess what their anathemas are. Hunting the people who once loved us is as easy as it gets. Our own faces work as a reminder of their former lives. We are the anathema.

So what am I? you may be asking.

I am parasite-positive, technically a peep, but I can still listen to Kill Fee and Deathmatch, watch a sunset, or put Tabasco on scrambled eggs without howling. Through some trick of evolution, I’m partly immune, the lucky winner of the peep genetic lottery. Peeps like me are rarer than hens’ teeth: Only one in every hundred victims becomes stronger and faster, with incredible hearing and a great sense of smell, without being driven crazy by the anathema.

We’re called carriers, because we have the disease without all the symptoms. Although there is this one extra symptom that we do have: The disease makes us horny. All the time.

The parasite doesn’t want us carriers to go to waste, after all. We can still spread the disease to other humans. Like that of the maniacs, our saliva carries the parasite’s spores. But we don’t bite; we kiss, the longer and harder the better.

The parasite makes sure that I’m like the always-hungry snail, except hungry for sex. I’m constantly aroused, aware of every female in the room, every cell screaming for me to go out and shag someone!

None of which makes me wildly different from most other nineteen-year-old guys, I suppose. Except for one small fact: If I act on my urges, my unlucky lovers become monsters, like Sarah did. And this is not much fun to watch.

Dr. Rat showed up first, like she’d been waiting by the phone.

Her footsteps echoed through the ferry terminal, along with a rattling noise. I left Sarah’s side and went out to the balcony. Dr. Rat had a dozen folding cages strapped to her back, like some giant insect with old-lady hair and unsteady metal wings, ready to trap some samples of Sarah’s brood.

“Couldn’t wait, could you?” I called.

“No,” she yelled up. “It’s a big one, isn’t it?”

“Seems to be.” The brood was still behind me, quietly attending to its sleeping mistress.

She looked at the half-fallen staircase with annoyance. “Did you do that?”

“Um, sort of.”

“So how am I supposed to get up there, Kid?”

I just shrugged. I’m not a big fan of the nickname “Kid.” They all call me that at the Night Watch, just because I’m a peep hunter at nineteen, a job where the average age is about a hundred and seventy-five. All peep hunters are carriers. Only carriers are fast and strong enough to hunt down our crazy, violent cousins.