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 most important known risk factor for Alzheimer's disease. The number of people with the disease doubles every 5 years beyond age 65.
Family history is another risk factor. Scientists believe that genetics may play a role in many Alzheimer's disease cases. For example, familial Alzheimer's disease, a rare form of Alzheimer's disease that occurs between the ages of 30 and 60, can be inherited. However, in the more common form of Alzheimer's disease, which occurs later in life, no obvious family pattern is seen. One risk factor for this type of Alzheimer's disease is a protein called apolipoprotein E, or apoE.
Everyone has apoE, which helps carry cholesterol in the blood. The apoE gene has three forms. One form seems to protect a person from Alzheimer's disease, and another form seems to make a person more likely to develop the disease. Other genes that increase the risk of Alzheimer's disease or that protect against Alzheimer's disease probably remain to be discovered.
Scientists still need to learn a lot more about causes and risk factors. In addition to genetics and apoE, they are studying education, diet, environment, and viruses to learn what role they might play in the development of this disease.
