Time cannot be the sole culprit behind a withering mind when most people remain sharp as rapiers well into their senior years.
Genetic mutations can increase the risk of Alzheimer's disease in some cases, and people with Down syndrome, who carry an extra chromosome, almost always develop plaques and tangles associated with Alzheimer's.
But Alzheimer's is considered sporadic and perplexing, and only in the mid-1970s did neuro-scientists recognize the disease as a common cause of death.
Suzanne de la Monte, neuropathologist at Brown University, feels an insulin problem in the brain, and not plaques, drives the disease. But she suspects that, even before that stage, something in the environment triggers the chain of neurodegeneration.
"I think it's something we are doing to ourselves," she says, "and if we can figure out what it is, we can undo it."
Dr. de la Monte says she has been convinced by her own observations. Back in the 1980s, she studied the brains of Asians who had died in their 80s and 90s, and noted they were remarkably free of the plaques associated with aged brains in the West.
"They were perfectly clean back then," she says. But now, just two decades later, the brains of elderly Asians also are freckled with sticky plaque build-up.
At the same time, she notes that diabetes - with strong links to fatty diets and obesity - has become one of the fastest growing diseases in China - and asks: "Is that a coincidence?"