The fire door opened behind me, and Mae said, "Ready when you are, Jack."

"Then let's do it."

We set off toward the rabbit, feet crunching on the desert sand. We moved away from the building. Almost immediately, my heart began to pound, and I started to sweat. I forced myself to breathe deeply and slowly, working to stay calm. The sun was hot on my face. I knew I had let Ricky spook me, but I couldn't seem to help it. I kept glancing toward the horizon. Mae was a couple of steps behind me. I said, "How're you doing?"

"I'll be glad when it's over."

We were moving through a field of knee-high yellow cholla cactus. Their spines caught the sun. Here and there, a large barrel cactus stuck up from the floor like a bristling green thumb. Some small, silent birds hopped on the ground, beneath the cholla. As we approached, they took to the air, wheeling specks against the blue. They landed a hundred yards away. At last we came to the rabbit, surrounded by a buzzing black cloud. Startled, I hesitated a step.

"It's just flies," Mae said. She moved forward and crouched down beside the carcass, ignoring the flies. She pulled on a pair of rubber gloves, and handed me a pair to put on. She placed a square sheet of plastic on the ground, securing it with a rock at each corner. She lifted the rabbit and set it down in the center of the plastic. She unzipped a little dissection kit and laid it open. I saw steel instruments glinting in sunlight: forceps, scalpel, several kinds of scissors. She also laid out a syringe and several rubber-topped test tubes in a row. Her movements were quick, practiced. She had done this before.

I crouched down beside her. The carcass had no odor. Externally I could see no sign of what had caused the death. The staring eye looked pink and healthy. Mae said, "Bobby? Are you recording me?"

Over the headset, I heard Bobby Lembeck say, "Move your camera down."

Mae touched the camera mounted on her sunglasses.

"Little more ... little more ... Good. That's enough."

"Okay," Mae said. She turned the rabbit's body over in her hands, inspecting it from all sides. She dictated swiftly: "On external examination the animal appears entirely normal. There is no sign of congenital anomaly or disease, the fur is thick and healthy in appearance. The nasal passages appear partially or entirely blocked. I note some fecal material excreted at the anus but presume that is normal evacuation at the time of death." She flipped the animal onto its back and held the forepaws apart with her hands. "I need you, Jack." She wanted me to hold the paws for her. The carcass was still warm and had not begun to stiffen.

She took the scalpel and swiftly cut down the exposed midsection. A red gash opened; blood flowed. I saw bones of the rib cage, and pinkish coils of intestine. Mae spoke continuously as she cut, noting the tissue color and texture. She said to me "Hold here," and I moved my one hand down, to hold aside the slick intestine. With a single stroke of the scalpel she sliced opened the stomach. Muddy green liquid spilled out, and some pulpy material that seemed to be undigested fiber. The inner wall of the stomach appeared roughened, but Mae said that was normal. She ran her finger expertly around the stomach wall, then paused. "Umm. Look there," she said.

"What?"

"There." She pointed. In several places the stomach was reddish, bleeding slightly as if it had been rubbed raw. I saw black patches in the midst of the bleeding. "That's not normal," Mae said. "That's pathology." She took a magnifying glass and peered closer, then dictated: "I observe dark areas approximately four to eight millimeters in diameter, which I presume to be clusters of nanoparticles present in the stomach lining," she said. "These clusters are found in association with mild bleeding of the villous wall."

"There are nanoparticles in the stomach?" I said. "How did they get there? Did the rabbit eat them? Swallow them involuntarily?"

"I doubt it. I would assume they entered actively."

I frowned. "You mean they crawled down the-"

"Esophagus. Yes. At least, I think so."

"Why would they do that?"

"I don't know."

She never paused in her swift dissection. She took scissors and cut upward through the breastbone, then pushed the rib cage open with her fingers. "Hold here." I moved my hands to hold the ribs open as she had done. The edges of bone were sharp. With my other hand, I held the hind legs open. Mae worked between my hands.

"The lungs are bright pink and firm, normal appearance." She cut one lobe with the scalpel, then again, and again. Finally she exposed the bronchial tube, and cut it open. It was dark black on the inside.

"Bronchi show heavy infestation with nanoparticles consistent with inhalation of swarm elements," she said, dictating. "You getting this, Bobby?"

"Getting it all. Video resolution is good."

She continued to cut upward. "Following the bronchial tree toward the throat ..." And she continued cutting, into the throat, and then from the nose back across the cheek, then opening the mouth ... I had to turn away for a moment. But she continued calmly to dictate. "I am observing heavy infiltration of all the nasal passages and pharynx. This is suggestive of partial or full airway obstruction, which in turn may indicate the cause of death." I looked back. "What?"

The rabbit's head was hardly recognizable any longer, she had cut the jaw free and was now peering down the throat. "Have a look for yourself," she said, "there seems to be dense particles closing the pharynx, and a response that looks something like an allergic reaction or-" Then Ricky: "Say, are you guys going to stay out much longer?"

"As long as it takes," I said. I turned to Mae. "What kind of allergic reaction?"

"Well," she said, "you see this area of tissue, and how swollen it is, and you see how it's turned gray, which is suggestive-"

"You realize," Ricky said, "that you've been out there four minutes already."

"We're only out here because we can't bring the rabbit back," I said.

"That's right, you can't."

Mae was shaking her head as she listened to this. "Ricky, you're not helping here ..."

Bobby said, "Don't shake your head, Mae. You're moving the camera back and forth."

"Sorry."

But I saw her raise her head, as if she was looking toward the horizon, and while she did so, she uncorked a test tube and slipped a slice of stomach lining into the glass. She put it in her pocket. Then looked back down. No one watching the video would have seen what she did. She said, "All right, we'll take blood samples now."

"Blood's all you're bringing in here, guys," Ricky said.

"Yes, Ricky. We know."

Mae reached for the syringe, stuck the needle into an artery, drew a blood sample, expelled it into a plastic tube, popped the needle off one-handed, put on another, and drew a second sample from a vein. Her pace never slowed.

I said, "I have the feeling you've done this before."

"This is nothing. In Sichuan, we were always working in heavy snowstorms, you can't see what you're doing, your hands are freezing, the animal's frozen solid, can't get a needle in ..." She set the tubes of blood aside. "Now we will just take a few cultures, and we're done ..." She flipped over her case, looked. "Oh, bad luck."

"What's that?" I said.

"The culture swabs aren't here."

"But you had them inside?"

"Yes, I'm sure of it."

I said, "Ricky, you see the swabs anywhere?"

"Yes. They're right here by the airlock."

"You want to bring them out to us?"

"Oh sure, guys." He laughed harshly. "No way I'm going out there in daylight. You want 'em, you come get 'em."

Mae said to me, "You want to go?"

"No," I said. I was already holding the animal open; my hands were in position. "I'll wait here. You go."

"Okay." She got to her feet. "Try and keep the flies off. We don't want any more contamination than necessary. I'll be back in a moment." She moved off at a light jog toward the door. I heard her footsteps fade, then the clang of the metal door shutting behind her. Then silence. Attracted by the slit-open carcass, the flies came back in force, buzzing around my head, trying to land on the exposed guts. I released the rabbit's hind legs and swatted the flies away with one hand. I kept myself busy with the flies, so I wouldn't think about the fact that I was alone out here.

I kept glancing off in the distance, but I never saw anything. I kept brushing away the flies, and occasionally my hand touched against the rabbit's fur, and that was when I noticed that beneath the fur, the skin was bright red.

Bright red-exactly like a bad sunburn. Just seeing it made me shiver.

I spoke into my headset. "Bobby?"

Crackle. "Yes, Jack."

"Can you see the rabbit?"

"Yes, Jack."

"You see the redness of the skin? Are you picking that up?"

"Uh, just a minute."

I heard a soft whirr by my temple. Bobby was controlling the camera remotely, zooming in. The whirring stopped.

I said, "Can you see this? Through my camera?"

There was no answer.

"Bobby?"

I heard murmurs, whispers. Or maybe it was static.

"Bobby, are you there?"

Silence. I heard breathing.

"Uh, Jack?" Now it was the voice of David Brooks. "You better go in."

"Mae hasn't come back yet. Where is she?"

"Mae's inside."

"Well, I have to wait, she's going to do cultures-"

"No. Come in now, Jack."

I let go of the rabbit, and got to my feet. I looked around, scanned the horizon. "I don't see anything."

"They're on the other side of the building, Jack."

His voice was calm, but I felt a chill. "They are?"

"Come inside now, Jack."

I bent over, picked up Mae's samples, her dissection kit lying beside the rabbit carcass. The black leather of the kit was hot from the sun.

"Jack?"

"Just a minute ..."

"Jack. Stop fucking around."

I started toward the steel door. My feet crunching on the desert floor. I didn't see anything at all.

But I heard something.

It was a peculiar low, thrumming sound. At first I thought I was hearing machinery, but the sound rose and fell, pulsing like a heartbeat. Other beats were superimposed, along with some kind of hissing, creating a strange, unworldly quality-like nothing I'd ever heard. When I look back on it now, I think that more than anything else, it was the sound that made me afraid.

I walked faster. I said, "Where are they?"

"Coming."

"Where?"

"Jack? You better run."

"What?"

"Run."

I still couldn't see anything, but the sound was building in intensity. I broke into a jog. The frequency of the sound was so low, I felt it as a vibration in my body. But I could hear it, too. The thumping, irregular pulse.

"Run, Jack."

I thought, Fuck it.

And I ran.

* * *

Swirling and glinting silver, the first swarm came around the corner of the building. The hissing vibration was coming from the cloud. Sliding along the side of the building, it moved toward me. It would reach the door long before I could.

I looked back to see a second swarm as it came around the far end of the building. It, too, moved toward me.

The headset crackled. I heard David Brooks: "Jack, you can't make it."

"I see that," I said. The first swarm had already reached the door, and was standing in front of it, blocking my way. I stopped, uncertain what to do. I saw a stick on the ground in front of me, a big one, four feet long. I picked it up, swung it in my hand. The swarm pulsed, but did not move from the door.

The second swarm was still coming toward me.

It was time for a diversion. I was familiar with the PREDPREY code. I knew the swarms were programmed to pursue moving targets if they seemed to be fleeing from them. What would make a good target?

I cocked my arm, and threw the black dissection kit high into the air, in the general direction of the second swarm. The kit landed on edge, and tumbled across the ground for a moment. Immediately, the second swarm began to go after it.

At the same moment, the first swarm moved away from the door, also pursuing the kit. It was just like a dog chasing a ball. I felt a moment of elation as I watched it go. It was, after all, just a programmed swarm. I thought: This is child's play. I hurried toward the door. That was a mistake. Because apparently my hasty movement triggered the swarm, which immediately stopped, and swirled backward to the door again, blocking my path. There it remained, pulsing streaks of silver, like a blade glinting in the sun. Blocking my path.

It took me a moment to realize the significance of that. My movement hadn't triggered the swarm to pursue me. The swarm hadn't chased me at all. Instead it had moved to block my way. It was anticipating my movement.

That wasn't in the code. The swarm was inventing new behavior, appropriate to the situation. Instead of pursuing me, it had fallen back and trapped me.

It had gone beyond its programming-way beyond. I couldn't see how that had happened. I thought it must be some kind of random reinforcement. Because the individual particles had very little memory. The intelligence of the swarm was necessarily limited. It shouldn't be that difficult to outsmart it.

I tried to feint to the left, then the right. The cloud went with me, but only for a moment. Then it dropped back to the door again. As if it knew that my goal was the door, and by staying there it would succeed.

That was far too clever. There had to be additional programming they hadn't told me about. I said into the headset, "What the hell have you guys done with these things?" David: "It's not going to let you get past, Jack."

Just hearing him say that irritated me. "You think so? We'll see." Because my next step was obvious. Close to the ground like this, the swarm was structurally vulnerable. It was a cluster of particles no larger than specks of dust. If I disrupted the cluster-if I broke up its structure-then the particles would have to reorganize themselves, just as a scattered flock of birds would re-form in the air. That would take at least a few seconds. And in that time I would be able to get through the door.

But how to disrupt it? I swung the stick in my hand, hearing it whoosh through the air, but it was clearly unsatisfactory. I needed something with a much bigger flat surface, like a paddle or a palm frond-something to create a large disrupting wind ... My mind was racing. I needed something.

Something.

Behind me, the second cloud was closing in. It moved toward me in an erratic zigzag pattern, to cut off any attempt I might make to run past it. I watched with a kind of horrified fascination. I knew that this, too, had never been coded in the program. This was self-organized, emergent behavior-and its purpose was only too clear. It was stalking me. The pulsing sound grew louder as the swarm came closer and closer.

I had to disrupt it.

Turning in a circle, I looked at the ground all around me. I saw nothing I could use. The nearest juniper tree was too far away. The cholla cactuses were flimsy. I thought, of course there's nothing out here, it's the fucking desert. I scanned the exterior of the building, hoping someone had left out an implement, like a rake ...

Nothing.

Nothing at all. I was out here with nothing but the shirt on my back, and there was nobody that could help me to-

Of course!

The headset crackled: "Jack, listen ..."

But I didn't hear any more after that. As I pulled my shirt over my head, the headset came away, falling to the ground. And then, holding the shirt in my hand, I swung it in broad whooshing arcs through the air. And screaming like a banshee, I charged the swarm by the door.

The swarm vibrated with a deep thrumming sound. It flattened slightly as I ran toward it, and then I was in the midst of the particles, and plunged into an odd semidarkness, like being in a dust storm. I couldn't see anything-I couldn't see the door-I groped blindly for the doorknob-and my eyes stung from the particles, but I kept swinging my shirt in broad whooshing arcs, and in a moment the darkness began to fade. I was dispersing the cloud, sending particles spinning off in all directions. My vision was clearing, and my breathing was still okay, though my throat felt dry and painful. I began to feel thousands of tiny pinpricks all over my body, but they hardly hurt.

Now I could see the door in front of me. The doorknob was just to my left. I kept swinging my shirt, and suddenly the cloud seemed to clear entirely away, almost as if it was moving out of range of my disruption. In that instant I slipped through the door and slammed it shut behind me. I blinked in sudden darkness. I could hardly see. I thought my eyes would adjust from the glare of sunlight, and I waited a moment, but my vision did not improve. Instead, it seemed to be getting worse. I could just make out the glass doors of the airlock directly ahead. I still felt the stinging pinpricks all over my skin. My throat was dry and my breathing was raspy. I coughed. My vision was dimming. I started to feel dizzy.

On the other side of the airlock, Ricky and Mae stood watching me. I heard Ricky shout, "Come on, Jack! Hurry!"

My eyes burned painfully. My dizziness grew rapidly worse. I leaned against the wall to keep from falling over. My throat felt thick. I was having difficulty breathing. Gasping, I waited for the glass doors to open, but they remained closed. I stared stupidly at the airlock. "You have to stand in front of the doors! Stand!"

I felt like the world was in slow motion. All my strength was gone. My body felt weak and shaky. The stinging was worse. The room was getting darker. I didn't think I could stand up on my own.

"Stand! Jack!"

Somehow, I shoved away from the wall, and lurched toward the airlock. With a hiss, the glass doors slid open.

"Go, Jack! Now!"

I saw spots before my eyes. I was dizzy, and sick to my stomach. I stumbled into the airlock, banging against the glass as I stepped inside. With every second that passed it was harder to breathe. I knew I was suffocating.

Outside the building, I heard the low thrumming sound start up again. I turned slowly to look back.

The glass doors hissed shut.

I looked down at my body but could barely see it. My skin appeared black. I was covered in dust. My body ached. My shirt was black with dust, too. The spray stung me, and I closed my eyes. Then the air handlers started up, whooshing loudly. I saw the dust sucked off my shirt. My vision was clearer, but I still couldn't breathe. The shirt slipped from my hand, flattening against the grate at my feet. I bent to reach down for it. My body began to shake, tremble. I heard only the roar of the handlers.

I felt a wave of nausea. My knees buckled. I sagged against the wall. I looked at Mae and Ricky through the second glass doors; they seemed far away. As I watched, they receded even farther, moving away into the distance. Soon they were too far away for me to worry any longer. I knew I was dying. As I closed my eyes, I fell to the ground, and the roar of the air handlers faded into cold and total silence.

DAY 6

11:12 A.M.

"Don't move."

Something icy-cold coursed through my veins. I shuddered.

"Jack. Don't move. Just for a second, okay?"

Something cold, a cold liquid running up my arm. I opened my eyes. The light was directly overhead, glaring, greenish-bright; I winced. My whole body ached. I felt like I'd been beaten. I was lying on my back on the black counter of Mae's biology lab. Squinting in the glare, I saw Mae standing beside me, bent over my left arm. She had an intravenous line in my elbow. "What's going on?"

"Jack, please. Don't move. I've only done this on lab animals."

"That's reassuring." I lifted my head to see what she was doing. My temples throbbed. I groaned, and lay back.

Mae said, "Feel bad?"

"Terrible."

"I'll bet. I had to inject you three times."

"With what?"

"You were in anaphylactic shock, Jack. You had a severe allergic reaction. Your throat almost closed up."

"Allergic reaction," I said. "That's what it was?"

"Severe one."

"It was from the swarm?"

She hesitated for a moment, then: "Of course."

"Would nano-sized particles cause an allergic reaction like that?"

"They certainly could ..."

I said, "But you don't think so."

"No, I don't. I think the nanoparticles are antigenically inert. I think you reacted to a coliform toxin."

"A coliform toxin ..." My throbbing headache came in waves. I took a breath, let it out slowly. I tried to figure out what she was saying. My mind was slow; my head hurt. A coliform toxin.

"Right."

"A toxin from E. coli bacteria? Is that what you mean?"

"Right. Proteolytic toxin, probably."

"And where would a toxin like that come from?"

"From the swarm," she said.

That made no sense at all. According to Ricky the E. coli bacteria were only used to manufacture precursor molecules. "But bacteria wouldn't be present in the swarm itself," I said. "I don't know, Jack. I think they could be."

Why was she so diffident? I wondered. It wasn't like her. Ordinarily, Mae was precise, sharp. "Well," I said, "somebody knows. The swarm's been designed. Bacteria's either been designed in, or not."

I heard her sigh, as if I just wasn't getting it.

But what wasn't I getting?

I said, "Did you salvage the particles that were blown off in the airlock? Did you keep the stuff from the airlock?"

"No. All the airlock particles were incinerated."

"Was that a smart-"

"It's built into the system, Jack. As a safety feature. We can't override it."

"Okay." Now it was my turn to sigh. So we didn't have any examples of swarm agents to study. I started to sit up, but she put a gentle hand on my chest, restraining me. "Take it slowly, Jack."

She was right, because sitting up made my headache much worse. I swung my feet over the side of the table. "How long was I out?"

"Twelve minutes."

"I feel like I was beaten up." My ribs ached with every breath.

"You had a lot of trouble breathing."

"I still do." I reached for a Kleenex and blew my nose. A lot of black stuff came out, mixed with blood and dust from the desert. I had to blow my nose four or five times to clear it. I crumpled the Kleenex and started to throw it away. Mae held out her hand. "I'll take that."

"No, it's okay-"

"Give it to me, Jack."

She took the Kleenex and slipped it into a little plastic bag and sealed it. That was when I realized how stupidly my mind was working. Of course that Kleenex would contain exactly the particles I wanted to study. I closed my eyes, breathed deeply, and waited for the throbbing in my head to ease up a little. When I opened my eyes again the glare in the room was less bright. It almost looked normal.

"By the way," Mae said, "Julia just called. She said you can't call her back, something about some tests. But she wanted to talk to you."

"Uh-huh."

I watched Mae take the Kleenex bag and put it inside a sealed jar. She screwed down the lid tightly. "Mae," I said, "if there's E. coli in the swarm, we can find out by looking at that right now. Shouldn't we do that?"

"I can't right now. I will as soon as I can. I'm having a little trouble with one of the fermentation units, and I need the microscopes for that."

"What kind of trouble?"

"I'm not sure yet. But yields are falling in one tank." She shook her head. "It's probably nothing serious. These things happen all the time. This whole manufacturing process is incredibly delicate, Jack. Keeping it going is like juggling a hundred balls at once. I have my hands full." I nodded. But I was starting to think that the real reason she wasn't looking at the Kleenex was that she already knew the swarm contained bacteria. She just didn't think it was her place to tell me that. And if that's what was going on, then she never would tell me. "Mae," I said. "Somebody has to tell me what's going on here. Not Ricky. I want somebody to really tell me."

"Good," she said. "I think that's a very good idea."

* * *

That was how I found myself sitting in front of a computer workstation in one of those small rooms. The project engineer, David Brooks, sat beside me. As he talked, David continuously straightened his clothes-he smoothed his tie, shot his cuffs, snugged his collar, pulled up the creases in his trousers from his thighs. Then he'd cross one ankle over his knee, pull up his sock, cross the other ankle. Run his hands over his shoulders, brushing away imaginary dust. And then start over again. It was all unconscious, of course, and with my headache I might have found it irritating. But I didn't focus on it. Because with every piece of new information David gave me, my headache got worse and worse.

Unlike Ricky, David had a very organized mind, and he told me everything, starting from the beginning. Xymos had contracted to make a micro-robotic swarm that would function as an aerial camera. The particles were successfully manufactured, and worked indoors. But when they were tested outside, they lacked mobility in wind. The test swarm was blown away in a strong breeze. That was six weeks ago.

"You tested more swarms after that?" I said.

"Yes, many. Over the next four weeks, or so."

"None worked?"

"Right. None worked."

"So those original swarms are all gone-blown away by the wind?"

"Yes."

"Which means the runaway swarms that we see now have nothing to do with your original test swarms."

"Correct ..."

"They are a result of contamination ..."

David blinked rapidly. "What do you mean, contamination?"

"The twenty-five kilos of material that was blown by the exhaust fan into the environment because of a missing filter ..."

"Who said it was twenty-five kilos?"

"Ricky did."

"Oh, no, Jack," David said. "We vented stuff for days. We must have vented five or six hundred kilos of contaminants-bacteria, molecules, assemblers." So Ricky had been understating the situation again. But I didn't understand why he bothered to lie about this. After all, it was just a mistake. And as Ricky had said, it was the contractor's mistake. "Okay," I said. "And you saw the first of these desert swarms when?"

"Two weeks ago," David said, nodding and smoothing his tie.

He explained that at first, the swarm was so disorganized that when it first appeared, they thought it was a cloud of desert insects, gnats or something. "It showed up for a while, going here and there around the laboratory building, and then it was gone. It seemed like a random event."