Dementia isn't a disease; it's a symptom. The term refers to a loss of brain function, as evidenced by memory loss, impaired judgment, behavior changes, learning difficulties, and communication problems. One in seven Americans over age 70 has some form of dementia, according to 2007 data from the nationally representative Health and Retirement Study -- but not all dementia is alike. It's caused by a variety of illnesses, some of which can be treated.

The number-one thing to do when someone exhibits memory loss or other mental or behavioral changes is to make sure the person gets a thorough medical evaluation.

Alzheimer's disease accounts for most cases of dementia -- 69.9 percent. In fact, the older the person, the more likely that the problem is due to Alzheimer's disease: Alzheimer's accounts for almost 80 percent of dementia in people age 90 or older, compared with just 46.7 percent among people in their 70s.

But other diseases, disorders, and medical conditions share similar symptoms and may be managed in different ways. Unlike Alzheimer's, some of these other dementias can be reversible.

Note: Early memory problems aren't always considered dementia. When they show up on memory tests but don't significantly affect daily living, mental impairments may reflect a lesser condition known as mild cognitive impairment (MCI).