BOSTON - Alzheimer's disease, the leading cause of dementia in older people, is brought on by the widespread destruction of brain cells called neurons over the span of several years.

As the destruction of neurons escalates, the ensuing brain damage slowly robs victims of the ability to learn, recall information, create and execute tasks. Judgment becomes impaired and sufferers become increasingly confused. At the end stages of the disease, Alzheimer's patients are generally bedridden and unable to communicate.

Because victims can become so incapacitated, Alzheimer's is often called a fatal illness. Actually, patients often die from complications associated with being bedridden. Poor communication skills often make it difficult for a patient to show being in distress, such as from a heart attack or serious infection.

Many doctors also suspect that the immune systems of Alzheimer's patients deteriorate during the course of the disease, which also can contribute to death.

Alzheimer's is the seventh leading cause of death in the United States.

According to the Alzheimer's Association, about 5.2 million American currently suffer from Alzheimer's. About 13 percent of all people aged 65 and older have the disease, with that number rocketing to about 50 percent at age 85.

The association predicts that by 2010, approximately 500,000 new cases of Alzheimer's will be diagnosed every year. By 2050, that number will leap to 1 million.

Many seniors with dementia actually suffer from a combination of Alzheimer's and vascular dementia, which is caused by decreased blood flow to the brain because of cardiovascular problems. Some may suffer only from vascular dementia. But Alzheimer's is still the leading cause of dementia in older people, accounting up to 80 percent of all cases.

While the scientific community is still searching for the exact cause of Alzheimer's, many believe that a brain protein called beta-amyloid is to blame. While younger brains are generally able to keep beta-amyloid levels in check, some older brains appear to lose that ability, allowing the protein to build up to toxic levels.

The results of the buildup are devastating, resulting in slow but steady destruction of neurons.

In the earlier stages of Alzheimer's, patients first experience problems with remembering recent events. Sufferers also begin to have problems recalling familiar words.

As the disease progresses, patients find it difficult going about their daily activities. Mathematical skills decline, along with the ability to recall key life events. Disorientation can also occur, with patients losing track of time or where they are.

Eventually self-help skills also erode, such as eating, dressing, washing and using the bathroom. Patients also run a higher risk of hurting themselves as they become increasingly unable to gauge dangerous situations. Personality changes and/or sudden mood swings are also common, and there can be a tendency to wander from home.

In the last stage of the disease, patients lose the ability to walk, communicate coherently and recognize loved ones. Swallowing can become difficult, and muscles can grow rigid.

Not unsurprisingly, Alzheimer's patients are also susceptible to mental illness as the disease worsens. Patients might become verbally or physically aggressive, anxious or depressed. In the latter stages of the disease, hallucinations, paranoia and delusional thinking can take hold. See related story: As boomers age, progress on Alzheimer's front

There are only four medications approved to treat the symptoms of Alzheimer's: Pfizer Inc.'s Aricept, Novartis AG's Exelon, Johnson & Johnson's Razadyne, and Forest Laboratories' Namenda.

The drugs are used to stoke the "firing power" of brain cells not destroyed by the disease, but can't stop its progression.