On the evening of June 10, 1806, Capt. David Hedges came home to find his wife, Phebe, ironing clothes. All seemed fine, and he went to bed. But the next morning, Phebe Hedges was gone.

Neighbors searched everywhere in the community of East Hampton, Long Island, where the Hedges were a prominent family of judges, silversmiths and whaling-ship captains, active in town politics and regular church-goers. They traced the woman's footsteps from her home through the fields and down to the shore. There the trail ended: Phebe Hedges had walked into the sea and drowned.

The town paper noted the death of Mrs. Hedges and speculated that she may have taken her own life because of her "extreme dread of the disorder" that had affected her mother and had started to manifest itself in her—the condition now known as Huntington's disease.

Huntington's disease is an inherited, fatal and degenerative neurological illness marked by involuntary movements, depression and cognitive decline. Like Phebe Hedges, every child of a parent with the disease has a 50-50 chance of inheriting it. The disease got its name from George Huntington, who, just out of medical school, described its features in an 1872 paper and laid out the key insight that it was a hereditary condition. Dr. Huntington based his famous paper on families he knew personally. His father and grandfather practiced medicine in East Hampton, where he grew up; his grandfather, in fact, was Phebe Hedges's doctor.

Phebe's death is the launching point for Alice Wexler's detailed and evocative portrait of a community now best known as a summer playground for the rich and famous. Using a range of sources—including diaries, letters, business ledgers, whaling logs and local newspapers—Ms. Wexler re-creates a picture of a long-ago place where doctors lived next-door to their patients and where generation after generation of a community's most prominent members struggled with a crippling illness.

Ms. Wexler's interest in this history is deeply personal. Her mother was diagnosed with Huntington's disease in 1968 and eventually died from it. Until then, Ms. Wexler did not know that her grandfather, uncles and cousins all had the disease, too. Her mother never spoke of it with her two daughters until her own diagnosis.

Ms. Wexler's father, Milton Wexler, went on to form the Hereditary Disease Foundation in 1968, today one of the best known and most successful of the patient-driven groups that have galvanized research and sped up the search for therapies in a range of once overlooked diseases. Ms. Wexler's sister, Nancy, was part of a team that in 1993 found the gene for Huntington's disease, a critical step in the effort to develop drugs that target it. Ms. Wexler wrote an earlier book (“Mapping Fate”) that chronicled her family's struggle with Huntington's and its role in creating a community of researchers, clinicians and patients searching for a cure. It is hardly surprising, given such a background, that she found herself interested in the family of Phebe Hedges and its own haunting history.

In a radio interview Ms. Wexler recently said that Phebe Hedges "walked into history when she walked into the sea." But Phebe left only a few historical traces behind—records in Dr. Huntington's account book that she sent for him, the mention in the town newspaper of her anxiety about possibly suffering her mother's fate. These tantalizing clues led Ms. Wexler to other sources and, ultimately, to what she described as a rewriting of the disease's narrative. She argues that Dr. Huntington's insights were possible in part because he carried with him a “community knowledge” of the disease. It went beyond the study of clinical symptoms to encompass, among other things, an understanding that all of us—the sick and the temporarily healthy—have genetic destinies stretching back in time and well into the future.

Ms. Wexler discovered that, ironically, an increased understanding of Huntington's disease actually discouraged doctors, early on, from pursuing a cure. Many thought that, because it was hereditary, nothing could be done. It was families like the Wexlers who felt otherwise and essentially transformed the way such research was pursued. They pushed forward the project to find the gene. They raised private money to fund research that mattered to them and required investigators to share their data and collaborate on projects with one another. They set up a company to drive the testing of drugs to see if any were effective against the disease. In doing so, they created a paradigm not only for understanding Huntington's disease but for targeting other diseases as well.

At the end of her book, Ms. Wexler describes the efforts of a congressional commission created in 1976—Nancy Wexler was its executive director—to study the needs of families with Huntington's disease and to draw up policy recommendations. A man whose family had been ravaged by Huntington's testified before the commission, telling it, poignantly, that his family had always thought they were alone. “Now,” he said, “we find that we're in a vast community,” and the community was trying to find a cure. This is one of the most important changes since the time of Phebe Hedges—the notion that, if any of us at some point will find ourselves facing the sea, we need not walk into it to hasten a grim fate. Something can be done.