The mud was black and smelled of copper. My father told me not to use it too much, and I didn't listen. I was twenty-six and confident, and confidence is just stupidity wearing a suit.

I mixed the mud with water and certain roots -- goldenseal, echinacea, a few others I'd learned from my grandmother -- and applied it to the wound on Josiah's leg. The wound was six inches long, running from his knee to his ankle, and it had been infected for three weeks. Three weeks of traditional treatments failing. Three weeks of my uncle rotting from the inside out.

The mud worked in twenty minutes. The swelling went down. The redness faded. The pus dried and crusted over and fell away. Josiah stared at his leg in disbelief. I stared at my hands in something like terror.

The Beauregard healing mud is not medicine. It never was. Medicine works by following principles -- Germ theory, inflammation response, tissue regeneration. The mud works by something else. Something older. When you mix it with the right roots and apply it to a wound, it doesn't heal the wound the way a doctor heals a wound. It heals the wound by pulling the health from somewhere else -- from the earth, from the air, from the blood of the person mixing it.

I learned this from my grandmother, before she died. She told me that the Beauregard family had been using the mud for 180 years, since her great-grandfather discovered it in the bottom of the bayou. She told me that every Beauregard healer who used it became healthier as their patients became healthier, until one day the healer became too healthy -- too much vitality, too much life force, and the body couldn't contain it.

That's when the black blood started.

Uncle Josiah is what happens when the black blood wins. I found him ten years ago, floating in a cypress swamp near the Deschanel plantation, half-consumed by something that had turned his veins black and his eyes yellow and his skin to the texture of wet clay. He was alive -- barely -- but he was no longer entirely human. He spoke sometimes, in a voice like water bubbling through reeds, saying things that weren't quite English and weren't quite any language I knew.

I brought him back to my cabin and treated him with the mud, and the mud kept him alive but didn't restore him. The black blood had gone too deep.

Now I was mixing more mud. More roots. More prayers to a God I wasn't sure was listening.

Rose Ann found me in the cabin. She stood in the doorway and looked at the black mud in the bowl and then at me and then at Josiah in the corner, whispering to the walls.

"Silas," she said. "You shouldn't be doing this."

"He's my uncle."

"He's a warning."

I mixed the mud with my hands. My fingers came away black. My veins were visible beneath the skin, dark and prominent, the black blood showing through at the wrists. I was thirty-eight, and I looked forty-five. The more I used the mud, the faster I aged. The faster I aged, the more desperate I became to use more mud, which made me age faster, which made me more desperate.

"How long?" Rose Ann asked.

"How long what?"

"How long until you're like him?"

I didn't answer. I went back to mixing.

She came into the cabin and stood beside me. Her hand was on my arm, and her touch was warm and real and human, and I felt something I hadn't felt in years: the urge to cry.

"I know what it is," she said quietly. "I know about the Deschanel plantation."

I stopped mixing. "What do you know?"

"My grandmother told me. Before she died. About 1837. About what happened in the cellar."

The cellar of the Deschanel plantation was a place the Beauregards and the Deschanels had agreed never to speak of. Two families, two lineages, connected by a secret that neither wanted to acknowledge.

"Tell me," I said.

"My grandmother said that in 1837, the Deschanel family discovered something in their cellar -- a hole in the earth, deep underground, filled with black mud. The mud could heal any wound, cure any disease, restore any body to perfect health. The Beauregards knew about it because they'd been searching for it for years -- looking for the healing source that would make their family famous and powerful."

"And what happened?"

"They used it. Too much. The Deschanel doctor -- your great-great-grandfather -- used the mud to heal his wife, who was dying of tuberculosis. He healed her. She became healthy beyond anything anyone had ever seen. And then she began to change. Her skin hardened. Her blood turned black. Her mind —" She stopped.

"Her mind what?"

"She became like Uncle Josiah. But worse. She ate her family. The Deschanel family buried her in the cellar, beneath the mud, and they sealed the cellar, and they agreed never to speak of it again. But the mud was still there. And the Beauregards took some of it with them, and they've been using it ever since."

I looked at my hands. The black veins ran up to my elbows. I could feel the black blood moving through me, strong and fast and hungry.

"Uncle Josiah used too much," I said.

"Everyone does," Rose Ann said. "That's not a curse, Silas. That's what the mud is. It gives you everything -- health, vitality, healing power -- and the price is yourself. The more you give, the less you are."

I looked at Josiah. He was whispering to the walls, his eyes open but not seeing, his mouth moving in a rhythm that wasn't language.

"What do I do?" I asked.

"You have a choice," she said. "You can stop using the mud. Your healing power will fade. Your black blood will stabilize -- you won't get worse, but you won't get better either. You'll live the rest of your life as you are now: a doctor who can heal, but at a cost you can afford."

"And if I keep using it?"

"Eventually, you'll be like him. Or worse."

I thought about my patients. The people in the bayou who came to me when they were sick, when they were hurt, when they were desperate. The people who trusted me because I was a Beauregard and Beauregards healed people.

I thought about Josiah, whispering to the walls, half-alive, half-human.

I thought about my father, who had died trying to find a way to use the mud without being consumed by it. A way that didn't exist.

I stood up. I walked to the corner of the cabin, picked up a hammer, and smashed the bowl of black mud. The mud cracked and dried and turned to dust.

Josiah stopped whispering. He looked at me with his yellow eyes, and for a moment -- just a moment -- he was my uncle again. He nodded.

I picked up the pieces of the bowl and threw them into the fire. I sat down beside Rose Ann and held her hand. Outside, the cypress trees stood in the black water, and the cicadas sang their endless song, and the bayou breathed around us, patient and indifferent and full of things we would never understand.

I would be a doctor. I would heal without the mud. It would be harder. It would be slower. But it would be honest.

And when the black blood finally claimed me -- and it would, because it was in my genes now, in my father's genes and his father's before him -- I would meet it with something my father hadn't had: a reason to keep fighting.