A person with AD usually has a gradual decline in mental functions, often beginning with slight memory loss, followed by losses in the ability to maintain employment, to plan and execute familiar tasks, and to reason and exercise judgment. Communication ability, mood, and personality may also be affected. Most people who have AD die within eight years of their diagnosis, although that interval may be as short as one year or as long as 20 years. AD is the fourth leading cause of death in adults after heart disease, cancer, and stroke.

Between two and four million Americans have AD; that number is expected to grow to as many as 14 million by the middle of the 21st century as the population as a whole ages. While a small number of people in their 40s and 50s develop the disease (called early-onset AD), AD predominantly affects the elderly. AD affects about 3% of all people between ages 65 and 74, about 19% of those between 75 and 84, and about 47% of those over 85. Slightly more women than men are affected with AD, but this may be because women tend to live longer, and so there is a higher proportion of women in the most affected age groups.

The costs for caring for a person with AD is considerable. The annual cost of caring for one AD patient in 1998 was estimated as about $18,400 for a patient with mild AD, $30,100 for a patient with moderate AD, and $36,100 for a patient with severe AD. The annual direct and indirect costs of caring for AD patients in the United States was estimated to be as much as $100 billion. Slightly more than half of AD patients are cared for at home, while the remainder are cared for in a variety of health care institutions.