What causes Alzheimer's disease? "I've spent most of my professional life trying to answer that question," says neuroscientist Rudolph Tanzi, director of the McCance Center for Brain Health at Harvard-af liated Massachusetts General Hospital. It's an important question to answer, because Alzheimer's is the most common form of dementia in many countries. In the United States, at least 10% of people over age 65 have the disease. As with most diseases, Alzheimer's is probably due to a combination of inherited vulnerability, the presence of other conditions, and social and lifestyle factors. And now