Dementia isn't a disease but a set of symptoms -- such as memory loss and declines in abstract reasoning and attention -- caused by changes in the brain that affect daily functioning. Alzheimer's disease is one specific type of dementia.

It might help to think about it like those logic problems in school: All people with Alzheimer's have dementia, but not all people with dementia have Alzheimer's.

Alzheimer's disease is the most common cause of dementia, accounting for more than half of all cases. There are dozens of other causes, including:

• Vascular dementia (also called multi-infarct disease, caused by small strokes).
• The neurological diseases Lewy body dementia, Parkinson's disease, Pick's disease, and Huntington's disease.
• Rarer disorders of the brain's frontal lobes, known as frontotemporal dementia.
• Infections (including urinary tract infections and AIDS).
• Alcoholism or substance abuse.
• Medication interactions or side effects.
• Nutritional deficiency.
• Head injury.

Alzheimer's disease isn't reversible, though it can be treated to slow its progress. Some cases of dementia can be treated and cleared up, however. This is why a thorough evaluation is important to determine the reason for dementia symptoms.