I n November of 1765, a physician from a respected Venetian family died in the Campo Santi       Apostoli, near the Jewish ghetto. The cause of death, according to a contemporary parish account, was “an organic defect of the heart’s sack,” though this was probably just guesswork. But priests ordinarily wrote detailed descriptions only of diseases that caught their eye, and this report is one of the longest in that year. In addition to suffering from the symptoms that were recorded (breathing difficulties, eventual paralysis), it is likely that the doctor had been extraordinarily anxious, like a horse at full gallop, sweaty and prone to shaking. And that he had been exhausted, falling in and out of a light, dream-wracked sleep. Many of the doctor’s descendants, down to the present decade, have shown similar symptoms in the course of dying from a disease we now call fatal familial insomnia, or FFI, suggesting that the doctor may have been the earliest recorded case.

FFI is an autosomal dominant mutation, which means that a child of a parent with the disease has a 50% chance of getting it too. This Italian family is one of a handful in the world afflicted with FFI. The family members have a carefully charted family tree: They can trace their disease back maybe to the Venetian doctor and with certainty to a relative of his, an aristocrat named Giuseppe who lived in the Veneto in the mid-nineteenth century. From him the disease goes down to Vincenzo in the 1880s, Giovanni in the 1910s, Pietro in the 1940s and Assunta, Pierina and Silvano in the last three decades of the twentieth century.

Although obscure, FFI is an important disease, a key constituent of a cluster called prion diseases. The first known prion disease was scrapie, initially chronicled in sheep in eighteenth-century England. With scrapie, the affected sheep, suffering from an itch caused by neurological alterations, rubs its back against objects until it rips its wool off. The disease is always fatal. In the 1950s, researchers in Papua New Guinea came upon another prion disease, kuru, that nearly wiped out a primitive tribe. Then in the late 1980s, Britain awoke to mad cow disease, which killed hundreds of thousands of cattle and jumped species to humans who had eaten the infected meat. The human form of the disease, known as variant Creutzfeldt-Jakob disease (vCJD), has killed roughly 160 people.

Prion diseases are difficult to diagnose because they confound basic epidemiology. For one thing, the time from infection to first symptom is extremely long—there are still members of the New Guinea tribe dying of kuru 50 years after being infected. And unlike other infectious diseases, prion diseases leave no detectable trace of infection in the body—no increased white-blood-cell counts, no fever. Odder still is that the agent behind them seems not to be a virus or a bacterium but a protein that replicates itself without DNA or RNA.

Yet as small as the family of prion diseases is, understanding them has deepened our knowledge of a variety of more common diseases. Prions generally form malignant protein plaques in the brains of their victims, a pathology that also characterizes Alzheimer’s and Parkinson’s diseases. Prion researchers believe that if they can cure prion diseases such as FFI, they may have an excellent point of entry into curing those others.

The Italian family with the rare sleeping sickness is an educated one: Its members were nobles in the old Venetian Empire and, more recently, doctors, high-ranking military officers and businessmen. But with public success has come hidden suffering and fear. In 1770, Giuseppe (who was probably the doctor’s nephew) was born in the Veneto. He had eight children, five of whom died in infancy and one, Costante, who fell gravely ill in his teens. Costante likely had trouble sleeping and hallucinated, a symptom of the disease when it strikes younger victims. His death was followed in short order by Giuseppe’s own and then through the decades by dozens of relatives. The disease unleashed a long chain of misery down through the centuries. Because FFI usually strikes late in life, it does not, like many other lethal genetic defects, prevent reproduction, so each generation passes on the mutation to the next.