The second development concerns diagnosis. Until recently, measuring amyloid levels in living patients required expensive PET scans or invasive spinal taps. Blood tests developed over the past five years approach the accuracy of those methods. The F.D.A. cleared the first such test in May, 2025; three more have followed. “For the first time,” Gates wrote, “we have simple blood tests that tell you whether you have Alzheimer’s.”
A disease that could once be confirmed only after death is now, in Gates’s telling, as easy to detect as high cholesterol. Combined with drugs that promise to slow deterioration, such tests are raising hopes of achieving a long-sought goal: treating prospective patients early enough that symptoms never appear. The result has been to give new urgency to one of the most fiercely debated questions in contemporary medicine: What, exactly, is Alzheimer’s disease?
At Donald Kewman’s home, outside Sacramento, there’s a sticker that reads “ALL ARE WELCOME” beside the front door. Inside, the living room shows signs of having been colonized by grandchildren. Bookshelves and a Persian-style rug share space with toy trucks, a purple plastic kitchen, and an easel with a basketball hoop attached to its back. Kewman, a seventy-six-year-old retired clinical psychologist, is “supremely good with little kids,” his wife, Sandy, told me—“willing to voice all the stuffies in ways that they prefer.” Kewman told me, “I like babies.” They were attending a grandchild’s eighth-birthday party later that day.
Kewman was adopted as an infant, and his concerns about Alzheimer’s began in 2016, when he signed up for genetic testing through 23andMe. When his results came back, he was given the option of learning his genetic risk for late-onset Alzheimer’s. “Of course, being a curious guy, I said yes,” he told me, his hazel eyes alert behind brow-line rectangular glasses. It turned out that he carried two copies of a gene called APOE4, making him much more likely than the average person to develop Alzheimer’s dementia. “It took a while to sort of absorb that information,” he said. Several years later, he was informed that his brain contained a moderate accumulation of amyloid plaques.
Alzheimer’s, he told me, is “at the bottom of my desirable ways to die.” A Quaker from the age of seventeen, he worries less about what it might do to him than about what it might mean for his loved ones. He and Sandy do not have long-term-care insurance, and he knows that the disease can impose “a terrible burden on family,” one that “can deplete your assets pretty dang fast.”
Sandy spent more than twenty-five years as an intensive-care nurse and understands the difficulties that could come with the disease. Yet Kewman remains cognitively normal. “We’ve taken care of our bucket lists and keep making bucket lists,” she said. When I asked him whether he thought of himself as having Alzheimer’s, he paused. “It’s a good question,” he said. “I have some of the changes associated with Alzheimer’s disease.” He meant the amyloid deposits. “So, in some sense, I do.” He quickly qualified his answer: many people with those changes never develop the “full-blown disease,” with its deterioration of “memory, behavior, functioning, independence.”
I asked Sandy the same question. “No,” she said. Not long ago, most physicians would have agreed.
Since antiquity, people have known that old age can bring lapses of memory and judgment. Alzheimer’s disease, however, was named barely a century ago. Coca-Cola and the automobile predate it, and its boundaries have repeatedly been contested and rewritten. The story conventionally begins on November 26, 1901, in an asylum in Frankfurt, where the physician Alois Alzheimer examined Auguste Deter, a housewife in her early fifties who had undergone a rapid mental decline. She had become suspicious of her neighbors and accused her husband of adultery. A photograph of her from around this time suggests an overwhelming sense of weariness. Deep lines mark her forehead as her heavy-lidded eyes gaze down. Her thin hands clutch each other over a white tunic. What most struck Alzheimer was Deter’s “utter perplexity.” His notes preserve their exchanges:
