With one last adrenaline-fuelled burst of energy, I flexed my arms and lifted myself up. I couldn't see anything now because the other boat was so close, the stars above me were nearly gone. I was lifting. And then it wasn't just my own force bringing me up because someone else had hold of my arms and, Jesus, it hurt but it didn't matter because I was over the railing and lying on the deck, gasping in fear and shock. Haru's face, three inches from mine, looked like it had aged twenty years since I'd last seen it.

  "What..?" I said, but he didn't let me finish, just yanked on my arm – my injured arm, and this time I managed not to scream, biting down on my tongue until it bled – and dragged me as far away from the other boat as he could.

  "Don't touch them!" he screamed. "For fuck's sake, don't let them touch you!"

  But how the hell didn't you touch four people who were climbing onto a thirty-foot wide boat with you? And why not? Were they contagious? Christ, could we turn into what they were? I suddenly wished, fiercely and hopelessly, that Soren and Kelis were with us. Or if not them, at least one of their guns.

  I was unarmed and Haru didn't have anything more deadly than a 2H pencil, and the crew of our boat were sailors, not soldiers. I saw one of them now, wrenching open a lockbox under the tiller with desperate fingers. The youngest of the… things which had boarded our boat trotted over the deck towards him. I'd been half expecting them to shamble, like B-movie zombies, but these people were alive. Somehow, they were still alive.

  All three of the others were watching the sailor, heads tilted as if in idle curiosity. But they were leaving the boy to take him on alone.

  "What do you want?" I said, not expecting any sort of answer.

  The man with the one good eye and the one pustulent hole, turned to face me. "Nothing you'll give us willingly," he said with a light Spanish accent, a voice you could have heard on the street and not thought about twice.

  Even on the other side of the boat I could hear the sailor's teeth chattering with fear. The boy was almost within touching distance now and the sailor was still trying to cram the key into a lock that didn't seem to want to take it. I didn't think there was any way he'd get it open in time, but then the key snicked into place and the gun was out of the lockbox and in his hand. He might not have been a soldier, but the kid was standing right next to him. Even with his hands shaking so hard that he could barely hold the weapon, he managed to put three bullets straight into the boy's chest.

  The boy staggered back a few paces – then kept on coming. Not enough stopping power, a voice inside me that belonged to my husband said. Another part that was still the little girl who'd been afraid of the dark was gibbering in fear of the unnatural things that couldn't be killed. But I was a scientist and nothing was irrational, only yet to be understood. I'd seen soldiers walking around with injuries that should have laid them out cold, because the body's own anaesthetic had kicked in and they just didn't know how bad things were yet.

  But there were some injuries no one walked away from.

  "The head!" I shouted. "Aim for the head!" After I'd said it I let out a half-hysterical choked laugh because maybe we were in a zombie movie after all.

  The sailor turned to look at me, as if he was about to ask me if I was certain, and for a moment I wanted to kill him myself. Then he turned back round, the boy's hands were only inches from his throat, but the gun roared one final time and the target was right in front of him. He didn't miss. The bullet tore through the boy's left eye and exited messily out the back of his head. He let out one quick, surprised cough, a trickle of arterial blood from his nose joining the gush from his head – then dropped on top of the sailor like a marionette with its strings cut.

  The sailor screamed an almost unearthly wail of complete panic. I thought he must have been hurt in some way. Maybe the boy had been carrying a knife, though I hadn't seen it. But then he pushed the boy off him and shoved himself to his feet, his mouth still open as the scream went on and on. His whole face and his white t-shirt were drenched in the boy's blood, black and shiny in the moonlight. Infected, I realised. He thinks he's been infected.

  And by the time I'd realised that it was already too late, because the sailor turned wide, desperate eyes to us for just one second and then turned and leapt over the side of the boat. Another second later, and the remaining three infected turned their heads to us, moving in an eerie kind of unison.

  The sailor had taken the gun with him, out of reach into the depths.

  Still, I knew they could be killed now. Haru was huddled behind me, whimpering. His stock of courage seemed to have been entirely used up dragging me over the side. Now he was hugging the boom as if it might offer him some sort of comfort.

  The boom.

  I pushed Haru out of the way, not really caring when I heard his head crack against the deck. He swore viciously in Japanese. The boom was tied off – of course it was. They'd been trying to get away from a boat full of god knows what, but it was important to keep up maritime discipline.

  Fuck!

  How could a rope that thick be knotted that tightly? My fingers picked at it feverishly but all I seemed to be doing was unravelling it. Haru pulled himself up from the deck, groaning, and he must have realised what I was doing because his fingers started working alongside mine. Maybe he'd picked up a thing or two since he'd been serving Queen M because the knot finally began to loosen.

  But shit – shit! – they were spreading out, the three of them fanning across the deck. One towards us, two towards the other sailors cowering uselessly in the stern. There was no way I was going to get all of them. Then the boom was free. Haru and I heaved on it together, and for once things were going my way because it swung easily, quickly, well-oiled and beautifully counterbalanced. Even though I think they were expecting it they weren't expecting it so fast. It took one, then two of them, and swept them clean off the deck and into the water.

  "They'll come back!" Haru said. "They'll climb back onboard!"

  "Then stop them!" I screamed because, for fuck's sake, did I have to think of everything myself? And clearly I did because he was still standing there, looking baffled. "Get rid of the grappling hooks – or use one to hit them with if they try to climb the sides!"

  He nodded once and then again, jerkily, but he still just stood there. His eyes were so wide that I could see the whites all the way around them. Shock, I thought. But I didn't have any patience for that. I wasn't exactly feeling on top of my game either, but I intended to finish the night alive and I needed Haru moving and functioning to help me achieve that. I yanked his shoulder round to turn him in the right direction and then shoved him on his way. He stumbled, then kept on walking and I had to assume he'd do what I told him because there was still one more of them on board and I'd caught a flicker out of the corner of my left eye. I knew she was coming straight for me.

  I felt like a creature composed of pure adrenaline. My senses were hyped, the smell of the invaders almost making me gag, only made bearable by the background salt smell of sea water that seeped through everything. Like a vague buzzing in the back of my mind, a half-recalled memory, I felt the pain of the bruises on my side, and my dislocated shoulder, but they weren't enough to distract me.

  Later, I'd wonder if that was what my husband felt when he went on those missions he could never tell me anything about. He'd always said danger was a high and I'd thought yeah, that's the cliche, but really isn't danger just frightening? In that instant I understood that it was absolutely both. And this, this moment when my actions would decide whether I lived or died, was the purest of my life.

  The thing was smiling at me as she walked forward, not a sneer or a grimace of anger but a real grin. Up close I suddenly saw that her cheekbones had the same angles, her eyes the same tilt as the dead boy's. Her son, I thought, but she didn't seem to care that he was dead. She just seemed… happy.

  Her split tongue flickered out, lizard-like, through the open lips of her smile.

  I didn't have any weapons, not even a knife. The boom was over the other side of the boat now. Even if it swung back, it would hit me and not her. And if I let her touch me, it was all over. She walked forward and I walked back, a pace at a time. One step from her. One step from me. Two. Then three. I was nearly at the railing and after that there was nowhere else to go but into the water.

  "What do you want?" I said to her again. "I'm a doctor. I can help you with… whatever the hell it is that's wrong with you."

  That made her stop, just for a second. She held up the jagged stump of her right hand, eying the protruding bone as if she hadn't really noticed it before. Then she looked back at me. "But why would I want to change?" she asked in the sort of warm Jamaican accent that made you feel as if the words were hugging you. "Everything's perfect just the way it is."

  And then she took one last step forward, and instead of stepping back I stepped forward too. I put my hands on the lapels of her denim jacket and I dropped back, foot out and into her stomach. It had been years since I'd learnt this. Since he'd made me go to self-defence classes, then made me practice with him at home, again and again, because London's a dangerous place and he couldn't bear it if anything happened to me. But I guess he was right, that once you've learnt it you never forget. It seemed to take no effort at all to pull her over my head and then kick off with my heel and flip her over the railing. Less than a second later, I heard the splash as she hit the water.

  I lay there for a second, shaking. She hadn't touched me. No part of her had touched me, I was sure of it. But the adrenaline had burnt itself out, purpose served, and all I could feel now was the desperate, paralysing fear I should have been feeling earlier.

  It took us another forty-five minutes to reach the flagship. There was a radio on the boat but a rebounding bullet had taken it out and there was no way for us to let anyone else know what had happened.

  Haru and I spent the time keeping watch, peering uselessly into the dense night for signs of any more pursuers. The sailors had wanted to tip the infected boy's body overboard but I'd persuaded them not to. When we got back I needed to study it, figure out just what the hell was wrong. Grumbling, they complied. The neat routines of their work broken up by the wide detour they took around his body and the pool of blood spilling out of it.

  When we got back to the flagship I asked to be taken straight to Queen M, but it was near midnight and the only person I could find was Kelis, back from whatever mission she'd been out on. She told me that it could wait till morning. I needed to sleep or I'd be making no sense to anyone anyway.

  I let her lead me back to my room because I was exhausted – my whole body was one big ache – and also because she just hadn't seemed that surprised when I told her about the people who attacked us and what seemed to be wrong with them. I needed some time to think about what that meant.

  The morning dawned bright but cooler. I shivered when I went out on deck in my shorts and tank-top, squinting against the piercing light of the rising sun. The blue sky, the blue seas, the distant palm trees suddenly looked a whole lot less reassuring than they had when I'd first arrived. Trouble in paradise, and then some.

  Queen M was already on her throne in the empty pool, lounging back with one leg slung over an arm, looking like she hadn't a care in the world. She stood and smiled when I approached, and I guessed she'd been waiting for me. Only a few people were out at that time of the morning and they drifted away when they saw me.

  "They come from Cuba," she said when I was ten paces from her. "My people call them the Infected."

  That stopped me in my tracks. "Cuba?" I don't know why it surprised me, but there was something too known, too package-holiday about Cuba for it to be the source of that terrible affliction.

  But she nodded. "They don't make any effort to disguise it, their boats are easy enough to track."

  "And has anyone gone there to find out what's going on?"

  Her bright eyes narrowed. "Would you go?"

  I felt the throb of the deep bruises covering my legs and chest and I shook my head.

  "I sent some people, back when they first showed up," Queen M said. "Twenty-four went, five returned. Back then it wasn't the whole of the island. Now as far as we can tell it's everyone. And it's started to spread. They say there have been cases on Haiti, some of the other Greater Antilles. As for what causes it…" She shrugged.

  "But you're sure it's infectious?"

  "How else could it be spreading?" Her eyes were still staring into mine, weighing everything up. She knows I want out, I thought. And this is her way of getting me to stay.

  I sighed because, yeah, she might be manipulating me, but whatever it was that was coming out of Cuba was more important than my anger at her, or my desire to escape. The world just couldn't take another Cull. It would be the end of us. "I'm not just a doctor," I reminded her. "I'm a researcher. I was part of the team investigating the Cull, so I've picked up a thing or two. We brought one of the Infected back. I can do an autopsy on him if you like, get some blood work done, whatever you've got the equipment for. See what I can find out."

  She smiled like a cat that had just been given detailed directions to the creamery. It occurred to me then that I'd never been told why the flagship had moved while I'd been on St Kitt's, or why she'd so unexpectedly decided to give me the day off.

  What I knew now was that she was the kind of person who was more than happy to kill a sailor or two if it got her what she wanted.

  The lab was in the bow of the ship, tucked away behind the casino in one of those areas that Kelis and Soren had steered me carefully away from. I thought it might once have been a crew kitchen, the gleaming metal surfaces and sinks obviously original but the pipettes, Bunsen burners and centrifuges were more recent additions. As was the autopsy table right in the centre of the room.

  I had a sudden flash of it being used by Queen M for other purposes, living subjects, the runnels to carry away the blood at the sides a convenience when you were trying to extract information from someone you didn't want to die quite yet.

  The current occupant of the table was very definitely dead though. Now I could see him under the bright, halogen lights I realised he was even younger than I'd first thought, barely into his teens. There were three others in the room when Kelis and I arrived, white-coated and bending intently over their workstations, test-tubes and Petri dishes spread out in front of them like a particularly unappetising meal. I gestured at the corpse of the Infected. "Mind if I take a look?"

  The nearest scientists, a harried looking woman in her forties, shrugged. "He's all yours. I'm an agronomist, corpses aren't my thing."

  "We're both electrical engineers," the man beside her said, nodding over at a third man who was peering through a microscope at some kind of circuit board. "You're the crew's first pathologist."

  "Yeah," I said. "Except I'm not. I'm a doctor and a biochemist, but I haven't performed an autopsy since medical school."

  "At least you've done one," the first woman said. "I wouldn't have a clue where to start."

  Kelis hovered at my elbow, peering over at the body with open curiosity. "First one you've seen close up?" I guessed.

  "Yup," she said. "Queen M always told us to steer clear, leave them be. Only recently they started getting aggressive, coming after us."

  I looked down at the boy's body, the vacancy where his left eye had been, and lower, were something had cut into his chest. Now that he was naked I could see other wounds too: a chunk out of his left thigh, two toes hanging off and another two broken and sticking upwards at an impossible angle. It was easy enough to tell which injuries were the result of the confrontation on the boat and which had been around a while. The new ones weren't running with puss, oozing yellow and green into the surrounding flesh.

  I decided to take a look at the chunks missing from his legs and stomach first. The edges of the cuts had been blurred by swelling and infection, but on the leg there was one little area that had remained relatively unscathed and it told me everything I needed to know. "Teeth marks," I said to Kelis, pulling back on the flesh and standing aside so that she could get a clear view.

  She turned her head aside and made a face. Funny, you wouldn't think a woman doing her job could be squeamish. "Joder! You're saying they eat each other?"

  I shook my head. "Not human. Shark, I think, though I've never treated a shark attack victim, so I can't be one -hundred per cent sure."

  She held a hand over her nose in a futile attempt to ward off the smell of corruption and leaned a little closer. "Doesn't look like they did anything to it after the attack. There's no stitches, nothing. Why would anyone let an injury like that go untreated?"

  "Yeah." I looked at his stomach, sure now that the flesh had been torn in the same incident. The level of infection was consistent too, both injuries dating back a couple of weeks. "It's like the shark bit him; he fought it off, climbed out of the water and then carried on like nothing had happened."

  "But that's not possible, is it?"

  I shrugged. "Short term, sure, it's amazing what a flood of adrenaline can do for you. Long term – no, it shouldn't be. He should have been in agony."

  "Any sign of brain damage maybe?" She peered at the boy's head, what was left of it. "Something that might explain why he can't feel any pain?"

  She was quick. I needed to remember that, in my plans. I sawed the boy's skull open but the damage from the bullet was too extensive to make out any subtler trauma around it. "Brain damage might explain what happened to him, but not the rest of them. It's too much of a coincidence for them all to have suffered the same condition."

  After the brain I went for the other organs, cracking open the ribs to get at them, wincing as blood splashed back at me from the corpse. The gown and mask caught it all and the examination didn't tell me anything I could use. The state of his liver would suggest too much drinking, but alcoholism just wasn't going to explain the things I'd seen on that boat. I used a scalpel to slice off a sliver of it anyway, along with the heart and the lungs, but I wasn't really expecting to find anything. I thought Kelis was probably at least partly right: whatever was wrong with these people was wrong with their brains.