Susan Okie, an assistant clinical professor of family medicine at Georgetown University School of Medicine, is a poet and a former medical reporter and science editor at The Washington Post.

Imagine watching your father slowly weaken, waste and die from a brain disease that no doctor has been able to diagnose, then learning that a similar illness also struck your grandfather and great-aunt. Imagine calling distant relatives and figuring out that, for generations, this mysterious plague has been killing members of your family, showing its first symptoms in midlife. Will you be next?

That’s the horrific question faced by three middle-aged brothers early in “Mercies in Disguise,” a moving, suspenseful page-turner that’s likely to become a classic of medical storytelling. Unlike many books about scientific quests, in which authors use patients’ personal experiences like coats of bright paint, a thin veneer for luring readers to engage with the science, this wonderful book by New York Times science writer Gina Kolata keeps the compelling human story at center stage. Scientists and doctors are foils or supporting players in the tale of the Baxleys, a tragically afflicted South Carolina family whose suffering and courage are reminiscent of Job — or Cordelia.

Physicians consulted by the family have either shaken their heads in defeat or labeled the illness Alzheimer’s or Parkinson’s disease, although it isn’t typical of either. As Bill Baxley, a chemical engineer, loses his memory, mental sharpness and control of his muscles, his sons Tim and Buddy Baxley — both physicians and desperate for a diagnosis — perform a spinal tap on him and even cut off a tiny piece of skin for testing, only to learn later that the specimens have been lost or ruined by laboratories. But at Bill’s funeral in 1998, a chance comment by a guest reveals that Bill’s father had suffered similar symptoms: “I remember his daddy walking through the plant. He had to hang onto the machines,” recalls a co-worker who had known both men. A year later, Tim, Buddy and their brother Mike, a lawyer, are staring at the family tree they’ve constructed, tracing the disease back through five generations. Seeing the grim pattern of deaths, Mike recalled later, was enough to “tell us there is light, there is hope.”

"Mercies in Disguise: A Story of Hope, a Family's Genetic Destiny, and the Science That Rescued Them," by Gina Kolata ( St. Martin's)

Meanwhile, for several decades, a few researchers had been focusing on rare diseases that cause progressive brain degeneration with symptoms like those seen in the Baxley family. In two early chapters interwoven with the beginning of the Baxleys’ saga, Kolata recounts the discovery of kuru, a fatal disease found only in the Fore, an isolated group of people in the mountains of eastern New Guinea. In the late 1950s, American scientist Daniel Carleton Gajdusek — “grandiose, driven, a visionary”— trekked to remote villages to find and study kuru cases. The disease predominantly struck women and children, who customarily handled the brains and organs of dead kuru victims during Fore funerals. Gajdusek deduced that the disease was transmitted during these rituals — but how? The microscopic holes seen in the brains of kuru patients resembled those found in another rare disorder, Creutzfeldt-Jakob disease, and both illnesses began with trembling and loss of muscle control. He eventually proved that brain tissue from human victims of both diseases could transmit the disorders to chimpanzees, but the symptoms took years to appear. Lacking an infectious agent, Gajdusek proposed that each might be caused by a “slow virus.” In 1976, Gajdusek received the Nobel Prize for discovering a new family of human diseases.

Next, another scientist with an outsize ego, neurologist Stanley B. Prusiner, having found no evidence of viruses in the affected brains, suggested in 1982 that these bizarre ailments might instead be caused by an abnormally contorted brain protein molecule that behaved somewhat like a virus, gumming up the interior of cells by inducing similar contortions in healthy protein molecules it contacted. He dubbed these “infectious proteins” prions. Derided by many in the scientific community for believing that a protein could transmit a disease, Prusiner and a colleague, Karen Hsiao, eventually identified a mutation in the gene coding for the prion protein in the blood of a man with another of these brain disorders, known as GSS (short for Gerstmann-Straussler-Scheinker disease). Unlike kuru, GSS runs in families. In blood samples taken from members of one such family, a scientific colleague detected a mutated prion gene in every living member affected by the disease, while in those relatives who had escaped it, the mutation was absent. Prusiner’s once-outlandish theory had been proven true: A genetic mutation had been linked to an inherited brain disorder, caused by a defective protein.

In 1997, after an FBI investigation, Gajdusek was convicted of and imprisoned for sexually abusing a minor —reflecting a pattern of sexual activity with children that many suspected had begun years earlier, during his time in New Guinea. That same year, Prusiner won the Nobel Prize for identifying the causal mechanism of kuru and other so-called prion diseases, a discovery that would profoundly influence the Baxleys.

But the family’s drama had yet to unfold. Certain that their still-unidentified family disorder was inheritable, the brothers waited in suspense as they approached the age at which certain relatives had fallen ill. One day, Tim, a neurologist, read an article about a case of GSS. The patient’s symptoms — abnormal gait, loss of balance, dementia — sounded like his father’s. Suspecting a prion disorder, Tim tracked down a neuropathologist and learned of a test to diagnose GSS from the brain of someone who had died. When his father’s sister, Faye, was hospitalized, he immediately recognized the symptoms: frozen facial expression, shaking, inability to walk. He persuaded her to agree to an autopsy upon her death. In 2002, post-mortem tests on her brain confirmed that the Baxley family disease was, indeed, GSS.

That news posed new, frightening dilemmas for the brothers’ generation and for their children. It meant the adult Baxleys didn’t need to wait and wonder whether a tremble or a stumble was the first sign. A genetic test on a blood sample could disclose in advance which of them carried the mutated prion gene. Each member of this deeply religious, small-town Southern family now confronted the choice of whether to be tested — whether learning how one was to die would make life more precious or would be an unbearable burden. How the various members chose — and the consequences of their choices — is at the heart of Kolata’s story. For the younger Baxleys who had not yet had children, the decision also carried a tempting but terrifying trade-off: In exchange for living with the certainty of future illness, a woman who carried the defective gene (or the wife of a man who had the mutation) could undergo in-vitro fertilization to create embryos that could be genetically tested before being implanted in the uterus, thus ensuring that GSS would not affect their offspring.

The Baxley brothers and their wives and children, particularly Buddy’s daughter, Amanda, the central character, are fully realized personalities. The reader shares their terror, denial and ambivalence, rooting for them as they grapple with the possibility of a fearsome death. Kolata describes their everyday interactions with one another, shadowed by the threat of illness. She writes in a straightforward, conversational style, seeming to know intimate details of conversations, even what was going through people’s minds. Her narrative is seamless and authoritative. The reader shares each twist and turn of the Baxleys’ fates, which I will not disclose. More than anything I’ve read, this book conveys what it’s like to grapple with the possibility of inheriting a fatal disease, and the emotional turmoil of undergoing genetic testing.

As a medical reporter, the question that enthralled me while reading “Mercies in Disguise” was: How did Kolata get the story? Her book recalls two other classic tales of medical anthropology, Rebecca Skloot’s “The Immortal Life of Henrietta Lacks” and Anne Fadiman’s “The Spirit Catches You and You Fall Down,” each written by authors who spent several years embedded with families to absorb their experiences of illness and their encounters with the medical system. “Mercies in Disguise” is described as a work of narrative nonfiction, presumably signaling that the author has taken some artistic license to reconstruct dramatic scenes in her characters’ lives where she probably wasn’t present, such as the moment in a psychotherapist’s office when a young patient learns her genetic fate over the phone. I assume that to write this book, Kolata must have developed a relationship with the Baxleys that spanned years, gaining their trust and permission to allow her to probe intimate details of their illnesses, their marriages, their emotions. In the scientific chapters, which are vividly and efficiently written, Kolata carefully annotates her statements about the key scientists’ research, findings and claims. But the chapters about the Baxleys contain no such annotations. I assume she visited them and interviewed them repeatedly, perhaps recording hours of conversations, but the book touches on the relationship only in a brief paragraph in the acknowledgments section, where Kolata, in thanking the family, writes, “When I began this book, the Baxleys were strangers to me, and I to them. But they opened their lives, patiently answering questions that forced them to relive some of the most painful experiences possible. They sent me photos and videos of doctor visits. They invited me into their homes.”

The lack of a wider window into the writer’s own process, particularly the omission of details about how the Baxleys came to welcome Kolata into their lives, and how sharing their journey affected her own thinking about mortality and life’s meaning, is the one flaw I found in this remarkable work.

Mercies in Disguise
A Story of Hope, a Family’s Genetic Destiny, and the Science That Rescued Them

By Gina Kolata

St. Martin’s. 262 pp. $25.99