A disease marked by the loss of cognitive ability, generally over a period of 10 to 15 years, and associated with the development of abnormal tissues and protein deposits in the cerebral cortex or a disease of the brain.

Bungled genetic instructions can cause proteins to tangle and to build up inside or in between nerve cells.

The result is that neurons die, leading to memory loss, dementia, and eventually death.

Certain genes increase the odds of Alzheimer's, but the most critical known risk factor is simply old age.

About 4.5 million Americans have the disease. That number is expected to triple to more than 13.2 million by 2050, when 20 percent of the U.S. population will be over 65.

Alzheimer's is the seventh leading cause of death in the United States.