Thomas Beauregard was twenty-nine years old when the snake bit him. He was a doctor at a small clinic in Baton Rouge, Louisiana, and he had spent the previous six months pursuing a group of moonshiners who had been poisoning the bayou with their chemical runoff. The moonshiners were armed, and Thomas knew it. He had warned the sheriff. The sheriff had not listened.

The moonshiners had copperhead snakes. Not as weapons, exactly, but as a kind of natural defense. They kept the snakes in wooden crates and released them when they saw outsiders approaching the bayou. Thomas had walked into one of those releases on a humid August night, and the snake had struck him on the left side of his face.

The bite was not fatal. Copperhead venom is rarely fatal to healthy adults. But it was painful, and it was ugly, and it left Thomas with permanent nerve damage on the left side of his face. His lip was pulled back in a permanent grimace. His left eye was half-closed by swelling. He looked like a man who had been in a fight with a demon and lost.

He went to every bar in Baton Rouge. He drank every bottle of whiskey they sold him. He fought every man who looked at him wrong. He tried to forget what he looked like. But every time he passed a mirror, he saw the man he had become, and he hated him.

He stopped going to the clinic. The other doctors offered him a desk job. He refused. He became a ghost in his own life, walking the bayou at night because during the day the sunlight was too bright and showed his face too clearly.

Then, in the autumn of 1922, he met Cousin Leona Fontenot.

She was seventy years old, thin and weathered like an old cypress tree, with hands that were rough from years of working in the earth and eyes that were sharp as flint. She was a bayou healer, one of the last of her kind, and she had inherited her knowledge from her grandmother, who had inherited it from her grandmother, going back six generations into the mists of time.

Thomas found her in a small cabin on the edge of the Atchafalaya Basin. She was sitting on her porch, grinding herbs in a mortar, and she looked up when he knocked on the door.

"Can I help you?" she asked.

"I'm Dr. Thomas Beauregard," he said. "I'm a doctor."

She looked at his face and did not flinch. "I can see that. Come in."

She made him tea from herbs that grew in her garden, and he told her his story. He told her about the snakebite, about his disfigured face, about the clinic he had left and the moonshiners he had been pursuing. He told her about the bars and the whiskey and the hating.

She listened without saying anything. When he finished, she poured herself another cup of tea and sat down across from him.

"You want to know how to fight poison," she said.

It was not a question. Thomas nodded.

"I've spent seventy years studying the herbal remedies of the bayou," she said. "They know something the modern medical establishment does not know. They know that the only way to fight poison is with poison. Not the same poison, never the same poison, but a smaller dose of a different poison that teaches the body how to fight the larger dose."

She went to a cupboard and brought out a collection of glass jars, each one containing a different dried herb or root or leaf. Snake root. Goldenseal. Black cohosh. She laid them out on the table like a dealer laying out cards.

"These are the remedies my grandmother taught me," she said. "They've been used in the bayou for generations to treat snakebites and poisonings and infections. They work because they contain compounds that fight the poisons that caused the damage in the first place."

She picked up a jar containing a dark brown powder. "This is snake root. It contains compounds that neutralize snake venom. If I give you a tiny dose, your body will produce antibodies. If you are then bitten by a snake, those antibodies will fight the venom before it kills you. This is the principle. Fighting poison with poison."

Thomas stared at the jar. He understood.

"Will you let me use myself as your subject?" he asked.

She looked at him for a long time. "It will be painful. Your face will swell. You may lose consciousness. But if it works, you will learn everything I know about bayou healing and toxicology, and you will be able to fight the poison that is coming to the bayou."

Thomas nodded. He had nothing left to lose.

The experiments began the next day and continued for two years. Thomas became Leona's student, her subject, her partner. They tested different herbs, different doses, different combinations. Thomas's face swelled and shrunk and swelled again. He lost consciousness dozens of times. He dreamed of snakes and fire and fog.

But slowly, slowly, his body learned. His blood produced antibodies. His nervous system adapted. He learned to read the symptoms of poisoning, to identify the type of venom, to calculate the correct antidote dose. He became, in Leona's words, a living encyclopedia of poison.

In the spring of 1924, a snakebite crisis hit the bayou. The copperhead population had exploded, and the snakes were coming out of the water and into the homes of the local families. People were being bitten by the dozens, and the bites were causing strange facial symptoms. Their faces swelled. Their skin turned gray. Some of them died.

The authorities did nothing. The state health department issued a statement that said the crisis was "a natural phenomenon" and advised citizens to "remain calm and avoid contact with snakes." They did nothing.

Thomas watched the crisis spread from Leona's cabin. He saw news reports of families boarding up their windows because the snakes were coming into their homes at night. He saw doctors overwhelmed with patients who had no treatment. He saw the bayou dying.

"I have to do something," he said.

Leona looked at him. "You know what you have to do."

Thomas nodded. He knew.

He went to the bayou that night and began his work. He collected venom from copperhead snakes, diluted it to tiny doses, and injected it into the people who had been bitten. He worked through the night, moving from house to house, injecting, observing, adjusting doses. Some patients survived. Some did not. But those who survived lived.

He became known as the Swamp Doctor. The residents of the bayou whispered about him. They said he was a doctor. They said he was a healer. They said he was a ghost. They were all wrong. He was just a man who had been bitten by a snake and refused to die.

For six months he worked. He treated hundreds of patients. He saved perhaps half of them. The other half died, and he carried their bodies to the cemetery at night and buried them in unmarked graves because he could not bear to see them left in the streets.

By the autumn of 1924, the crisis was over. The snake population had declined. The people who had been bitten and survived had developed immunity. The bayou was safe.

Thomas returned to Leona's cabin, and he sat on her porch and looked at his reflection in the window. His face was more scarred than ever. The constant swelling and shrinking had left his skin pitted and disfigured. He looked like a man who had been carved from stone by a drunkard's hand.

"Did it work?" Leona asked.

Thomas nodded. He had saved lives. He had fought poison with poison. He had won.

But the victory was incomplete. He could never go back to normal life. He would never be Dr. Thomas Beauregard, the clinic doctor, again. He would always be the Swamp Doctor, the man who fought poison with poison.

He sat in his cabin on the edge of the Atchafalaya Basin and looked at the moonlight on the water. The cicadas were singing, and the frogs were croaking, and the snakes were slithering through the reeds. The bayou was alive, and it did not know how close it had come to dying.

Thomas smiled. He had saved it. He had fought poison with poison. He had won.

And then he went back to work.

His grandson, Levi Fontenot, watched him go. Levi was twenty years old, and he had been Thomas's apprentice for the past two years. He had learned everything Thomas had taught him about bayou healing and toxicology. He had learned how to identify the herbs, how to calculate the doses, how to read the symptoms of poisoning.

When Thomas died, in the winter of 1925, alone in his cabin while the moonlight was on the water, Levi was there. He closed his master's eyes and carried his body to the cemetery and buried him in an unmarked grave. He placed a small stone on the grave with one word carved into it: Venom.

The residents of the bayou never forgot him. For years after his death, they would sometimes hear a voice in the fog, a broken, raspy voice that said the same words over and over: fighting poison with poison. fighting poison with poison. fighting poison with poison.

And sometimes, on the nights when the fog was thickest and the moonlight was hidden by clouds, they would see a gray face looking at them from the edge of the bayou, and they would know that Thomas Beauregard was still watching, still fighting, still breathing.

Levi wrote about him in his notebook. He wrote about the doctor who had been bitten by a snake and refused to die. He wrote about the man who fought poison with poison and won. He wrote about the Swamp Doctor.

Then he closed his notebook and went back to work.