The child was blue at the edges when I arrived. Not the blue of cold -- Clara had checked his temperature three times -- but a deeper color, a blue that came from within, as if his blood had forgotten how to carry oxygen through the simple fact of being afraid.

"His mother says it began after the market," Clara whispered, stepping aside so I could see him. The boy was seven, thin as a rail, lying on a pallet in the corner of the workhouse infirmary where I'd established my practice by circumstance rather by invitation. He couldn't speak. His mother, a washerwoman with raw hands and a face like cracked porcelain, stood by the door with her shawl pulled tight as if the very air of Whitechapel could infect her.

I knelt beside the boy. His pulse was there, weak but steady. His skin was cool. His eyes were open, unfocused, fixed on something I couldn't see. I placed two fingers on his wrist, then pressed the stethoscope to his chest. His heart beat -- slow, but regular.

"What is it?" Clara asked.

"I don't know yet," I said. And that was the truth, at least.

I had been a doctor for nine years -- nine years since my father died in the Essex marshes, dragging himself back from the reeds with black veins crawling beneath his skin and a single sentence on his lips: "Do not heal the sick beyond what they can bear." I was twenty then. I did not understand it. I still wasn't sure I did.

I examined the boy thoroughly -- listened to his lungs, checked his pupils, felt the soft places behind his ears. Everything was normal. The boy was breathing. His heart beat. His pulse was present. But he was not well, and the longer I stared at him, the more certain I became that his illness was not in his body.

"Give me water," I said.

Clara brought a bowl. I dipped a clean cloth in it and pressed it to the boy's forehead. He flinched, just slightly -- the first reaction I'd seen from him. Good. His nervous system was intact.

"I'll need to stay with him tonight," I told Clara. "And I need to see his mother."

The washerwoman came when I asked. She stood in the doorway and wrung her hands as I spoke.

"Your son is sick," I said carefully. "I can treat the symptoms. But I need to understand the context. When did this begin exactly?"

"Wednesday," she said. "The Wednesday market. He was running about, as boys do, and he fell near the fishmonger's stall. Cut his knee on something -- I don't know what. I cleaned it with vinegar and bandaged it with clean linen. It looked fine. But that night he stopped speaking."

"Did he have a fever?"

"No fever. No cough. He simply -- stopped."

I looked at the boy. He was watching me now. His eyes were dark, intelligent, and filled with a kind of exhausted resignation that no seven-year-old should possess.

"How are you feeling, William?" I asked him gently.

His lips moved but no sound came out. I placed my hand on his. His fingers closed around mine -- a reflex, or something more.

"I'll do what I can," I said.

I treated him as any physician would: with cool compresses, with warm broth, with cleanliness and patience. By morning he could speak again. By the end of the week he was sitting up, eating soup, pointing at the rats that infested the workhouse corridors. Clara was optimistic. She said I'd done what doctors were supposed to do.

I should have known better.

William recovered fully. He ran the streets of Whitechapel for three months, strong and healthy and irrepressible as any child. I saw him weekly at the workhouse, and each time he was well -- too well, almost. His color was ruddy. His pulse was strong. He was everything a cured patient should be.

Then came the autumn.

It started with the cough. A simple cough, nothing more, and I treated it with expectorants and rest. But the cough became something else -- a wet, rattling sound that made even Clara stop in the doorway and cross herself. By the time I realized what was happening, it was too late.

William's lungs filled with fluid. Not infection -- I checked for that. Not tuberculosis -- I examined him thoroughly. But his lungs were filling with something that shouldn't exist: water, pure and clean, produced by his own body as if some mechanism deep inside him had reversed its function.

"He'll need to be upright," I told his mother on the third night. "If he lies flat, he'll drown in his own body."

She sat by his pallet and held his hand, the way mothers hold the hands of their children when they know, even when they don't want to know. I sat with them. I was a good doctor. I knew medicine. I knew exactly what was happening and exactly how to treat it, and the treatment was useless.

William died at dawn, sitting upright in his mother's arms, drowning in a room full of dry air.

I documented everything. I wrote in my journal, in the hand I'd inherited from my father -- meticulous, clinical, precise. The boy's pulse, his temperature, the fluid accumulation rate, the timeline. I could not sleep after that night. I walked the streets of Whitechapel until the gas lamps dimmed and the first fishmongers began arriving at the docks, and I thought about my father's last words.

Do not heal the sick beyond what they can bear.

I had healed William completely. I had given him three months of perfect health after his illness. I had restored him to a state he had never known. And the bill came due in October.

I began reviewing my records. Every patient I had treated over the past nine years. I had been generous with my gift -- I could feel it in my hands, this ability to diagnose and heal that was both extraordinary and unexplainable. I had treated hundreds of people: dockworkers with fractured ribs, children with fever, old women with failing hearts. And each time, I had done my best. Each time, I had succeeded beyond what any ordinary physician could achieve.

But I had never tracked the outcomes. I should have.

I pulled the ledgers from the shelf. Three hundred and forty-seven patients. I began to sort them by date of last treatment and outcome. The first dozen were unremarkable -- people I had treated for minor ailments who remained well. But then I found the pattern.

Fifty-two people had died after their treatment. Not immediately -- never immediately. Always months, sometimes years later. Always in ways that were medically inexplicable. A woman whose treated wound reopened and never closed. A man whose heart -- previously strong as an ox's -- simply stopped without warning. A child whose recovered fever was followed by a wasting sickness that consumed him in weeks.

I closed the ledger and placed my hands on the desk. They were trembling.

My father had treated perhaps a thousand people in his lifetime. He had been considered the finest physician in Essex. And when he died, the marsh had claimed him -- not with cholera or fever or any common illness, but with something that turned his veins black and his blood thick and his mind to fog.

I understood now. Not everything -- not even most things -- but enough to walk to the window and look out at the Thames, black and moving and indifferent to the lives that depended on it.

There was a knock at the door. Clara stood there, holding a letter.

"A gentleman came asking for you," she said. "Said he had information about your father. About the marshes."

I took the letter. It was sealed with wax bearing a symbol I didn't recognize -- a spiral inside a circle, like a cell viewed through a microscope. I broke the seal and read the single sentence inside.

Dr. Blackthorne: The marsh remembers your father. It would like to meet you.

I looked up at Clara. She was waiting, and I realized I had been standing at the window for a long time, staring at the river, thinking about the three hundred and forty-seven people in my ledger and the one man in the marsh who was still waiting for an answer.

"Tell him I'll come," I said.

And for the first time in nine years, I felt the weight of the hands that had healed so many and destroyed them all, and I understood that my gift was not a gift at all, but a loan -- and every person I had cured was now a debtor to something ancient and patient and very, very hungry.

I sat down at my desk and opened a new page in my journal. I wrote my name. I wrote the date. And I began to list every name of every person I had ever treated, because if there was to be a reckoning, I wanted to be ready to face it -- not as a healer, but as a man who had unknowingly signed three hundred and forty-seven death warrants in the name of mercy.

The marsh would come for me next. I could feel it. It always came for the healers first.