Under a microscope, a pathologist searching through the damaged nerve cells in a brain tissue sample from a patient who has died of Alzheimer's disease can make out strange clumps of material. They consist of proteins that clearly do not belong there. Where did they come from, and why are there so many of them? And most important, what do they have to do with this devastating and incurable disorder? The search for answers has turned up a startling discovery: the clumped proteins in Alzheimer's and other major neurodegenerative diseases behave very much like prions, the toxic proteins that destroy the brain in mad cow disease.