The cause of Alzheimer's disease is currently unknown. It is not contagious. Genetic factors and aging appear to play an important role. Because a combination of factors are believed to be responsible for most forms of Alzheimer's disease, genetic testing usually is not recommended.

Alzheimer patients who have at least one other relative with the disease are categorized as ''familial.'' ''Familial'' does not necessarily mean that is it genetic; family members may have been exposed to something in the environment that caused the disease. If a person has Alzheimer's disease and not other family members are known to have been affected, they are said to have ''sporadic'' Alzheimer's disease.

As stated earlier, most cases of Alzheimer's disease occur in those after age 65, but a small percentage of cases develop at an unusually young age-some people are diagnosed in their fifties, some in their forties, some even as young as their thirties. This form of the disease is called early - onset Alzheimer's disease, affecting from 1 to 10 percent of all cases.

A variation on chromosome 19, called APOE-e4, appears to be a risk factor for Alzheimer's. This gene variation is present in about 15 percent of the general population, but occurs in 50 percent of those with late-onset Alzheimer's disease. It is more than three times as common in Alzheimer's patients than in people without the disease. Although people with this so-called e4 type appear to be more susceptible to the disease, they will not necessarily get it.