IF YOU WAKE UP one morning with a headache, you might assume that you drank one glass of wine too many the previous night, or that the heat was up too high, or that you are coming down with a cold. You are less likely to jump to the conclusion that you have a brain tumor. But when you find yourself forgetting things, then, omigosh, it must be the onset of Alzheimer's disease. Men and women who are middle-aged—or older, especially—are quick to diagnose themselves with inevitable dementia.