Alzheimer's disease (AD) is the most common cause of dementia in older people. Dementia is a brain disorder that makes it hard for a person to carry out normal daily activities. Symptoms of dementia include changes in memory, personality, and behavior.

There are other conditions besides AD that can cause dementia. For example, small strokes or changes in the brain's blood supply can cause multi-infarct or vascular dementia. Some causes of dementia are reversible, such as drug reactions, dehydration, and thyroid problems.

AD affects the parts of the brain that control thought, memory, and language. It is a slow disease that starts with mild memory problems and leads to severe brain damage. People with AD lose their abilities at different rates. AD can last from 3 to 20 years or more after the onset of symptoms. It is not yet clear what causes AD and there is no known cure.

An estimated 4 million people in the U.S. suffer from AD. The disease usually begins after age 60, and the risk of AD goes up with age. However, some cases of AD occur in younger people. About one in ten persons over 65 have AD, and nearly half of those age 85 and older may have the disease. But AD is not a normal part of aging.

AD is named after Dr. Alois Alzheimer, a German doctor. In 1906, Dr. Alzheimer noticed changes in the brain tissue of a woman who had died of an unusual mental illness. He found abnormal clumps (now called neuritic plaques) and tangled bundles of fibers (now called neurofibrillary tangles). Today, these plaques and tangles in the brain are considered hallmarks of AD.

Scientists have found other changes in the brains of people with AD. There is a loss of nerve cells in areas of the brain that are vital to memory and other mental abilities. There also are lower levels of chemicals in the brain that carry complex messages back and forth among nerve cells. AD may disrupt normal thinking and memory by blocking these messages in the brain.

What causes Alzheimer's disease?

Scientists do not yet fully understand what causes AD. There probably is not one single cause, but several factors that affect each person differently. Age is the most important known risk factor for AD. 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 AD cases. For example, familial AD, a rare form of AD that usually occurs between the ages of 30 and 60, can be inherited. However, in the more common form of AD that occurs later in life, no obvious family pattern is seen.

Scientists still need to learn a lot more about what causes AD. In addition to genetics, they are studying education, diet, environment, and infections to learn what role they might play in the development of this disease.