Alzheimer's disease is a progressive deterioration in mental function. Brain cells become filled with tangles of protein fibers (and subsequently die). The connections between brain cells are also disturbed by the accumulation of deposits of a different protein. It is unclear what causes Alzheimer's disease, but it is abundantly clear how terrible and how common it is. About 5 million people in the United States are thought to suffer from it, and an additional 19 million or so are affected in the rest of the world. Although treatments are available to slow the progression of this illness, nothing is known to cure it or prevent it.