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.