Alzheimer's disease affects both sexes (although it is slightly more common in women) and all ethnic groups. The illness is common; approximately four million Americans have Alzheimer's disease. The risk of developing the illness increases as an individual ages. Most often the first symptoms develop when patients are in their 70's, but they can begin as early as age 40 or as late as age 90. Nevertheless, Alzheimer's disease is uncommon under the age of 60. This is evident when looking at the prevalence of the disease by age. While only three to five percent of all individuals who are 65 years old have Alzheimer's disease, by age 75 the prevalence increases to 20 percent and by age 85 it reaches 40 percent.

An increased risk for Alzheimer's disease has been linked to a specific gene called ApoE/e4 (a form of the alpolipoprotein E gene). The presence of ApoE/4 is a risk factor, not a direct cause of Alzheimer's disease; one can have this risk factor and not get the disease. There is also an increased occurrence of Alzheimer 's Disease in people who have a first-degree relative (parent, brother, or sister) who has had the disease.