John C. Morris, MD, FAAN, responds:

Increasing age and a family history of Alzheimer's disease are still the two greatest risk factors. A family history is more often associated with developing the disease earlier, between the ages of 30 and 60, but by far the most frequent form of Alzheimer's disease appears after age 70.

The likelihood of developing the disease because a parent has it depends on whether you inherit a copy of the apolipoprotein e4 (APOE e4) gene, the major genetic risk factor for developing the disease after age 70. If you do inherit a copy of the APOE e4 gene, your susceptibility for Alzheimer's disease is at least double that of the general population. In the rare case that you inherit copies of APOE e4 from both parents, your susceptibility jumps almost sixfold. Research has uncovered other susceptibility genes, but none is as strong a risk factor as APOE e4. In all cases, however, having the genetic risk factor does not cause the disease, nor does it guarantee you will inherit it, but it does increase your susceptibility.

Rare Genetic Defects

Less than 1 percent of all cases of Alzheimer's disease are caused by a defect, or mutation, in one of three distinct genes: presenilin 1, presenilin 2, and the amyloid precursor protein. Each biological child of a person with Alzheimer's disease caused by one of these mutations has a 50-50 chance of inheriting the mutation. Those who inherit it are destined to develop the disease, usually at an early age-on average, around 45 years old. Those who do not inherit the mutation are no more at risk than anyone else.

Risk Modifiers

Susceptibility genes such as APOE e4 explain why Alzheimer's disease appears more frequently in some families, but these genes do not necessarily cause the disease. Moreover, half of all people with Alzheimer's disease lack the APOE e4 gene, so other factors are involved.

People understandably want to lower their risk. For now, the best advice is to maintain good health, particularly cardiovascular health. Manage conditions such as hypertension, diabetes, and high cholesterol; quit smoking; follow a heart-healthy diet; and stay engaged physically, mentally, and socially.

Dr. Morris is the Harvey A. and Dorismae Hacker Friedman Distinguished Professor of Neurology at Washington University in St. Louis, MO. He is also director of the Charles F. and Joanne Knight Alzheimer's Disease Research Center, a Fellow of the American Academy of Neurology, and a member of the Neurology Now editorial advisory board.