Alzheimer's disease is a progressive disorder that causes brain cells to waste away (degenerate) and die. Alzheimer's disease is the most common cause of dementia — a continuous decline in thinking, behavioral and social skills that disrupts a person's ability to function independently. Scientists do not yet fully understand what causes Alzheimer’s disease. There probably is not one single cause, but several factors that affect each person differently. Age is the best known risk factor for Alzheimer’s disease, Family history—researchers believe that genetics may play a role in developing Alzheimer’s disease. Changes in the brain can begin years before the first symptoms appear. Researchers are studying whether education, diet, and environment play a role in developing Alzheimer’s disease. Scientists are finding more evidence that some of the risk factors for heart disease and stroke, such as high blood pressure and high cholesterol may also increase the risk of Alzheimer’s disease. There is growing evidence that physical, mental, and social activities may reduce the risk of Alzheimer’s disease.