Among award-winning actress Gena Rowlands’s many film credits is The Notebook, the 2004 romantic drama in which she played a woman living with dementia. Now the 94-year-old star is herself living with Alzheimer’s disease, her son — and director of The Notebook — Nick Cassavetes, told Entertainment Weekly. “I got my mom to play older Allie, and we spent a lot of time talking about Alzheimer’s and wanting to be authentic with it, and now, for the last five years, she’s had Alzheimer’s,” he said.

According to Entertainment Weekly, Gena Rowland’s mother, actress Lady Rowlands, had the disease too. Here’s what to know about the leading cause of dementia in older adults and what having a family history of Alzheimer’s means for risk.

What is Alzheimer’s disease?

Alzheimer’s disease affects an estimated 6.7 million people in the U.S., according to the Alzheimer’s Association, while a 2020 survey estimated that about a third of Americans have a close relative with Alzheimer’s or another form of dementia.

Alzheimer’s is one of several types of dementia, a disease that interferes with a person’s memory, behavior and ability to think clearly. It accounts for between 60% and 80% of dementia diagnoses in the U.S..

Alzheimer’s is distinguished from other dementia and memory loss primarily by how it manifests in the brain, Dr. Kostas Lyketsos, director of the Johns Hopkins Memory and Alzheimer’s Treatment Center, tells Yahoo Life. The tell-tale signs are plaques of a byproduct of a protein, called amyloid beta, and tangles made up primarily of another protein, called tau. Lyketsos says that doctors like himself can usually distinguish between Alzheimer’s and other forms of mild cognitive impairment by the time symptoms show up, but the differences are subtle. Symptoms of Alzheimer’s tend to be more severe and may worsen more quickly than other forms of dementia or cognitive impairment. “Short-term memory difficulties — leading a person to ask repeated questions or forget recent events or conversations — are frequently some of the earliest symptoms” of Alzheimer’s, Dr. Vijay Ramanan, a Mayo Clinic neurologist, tells Yahoo Life. Other forms of dementia often present first with different symptoms, including personality changes.

What causes the disease?

Scientists are “not 100% clear” on causes, says Lyketsos, and says that a family’s genes are a major predictor of their Alzheimer’s risk. “But there are really two kinds of genetic relationships,” he says. Ramanan emphasizes that “having a family history of the disease does increase one’s personal risk to a degree, but it is important to remember that nearly all cases of Alzheimer’s disease are not directly caused by a gene change.”

There are three genes that are “deterministic,” however, Lyketsos says. “If you get one of these bad alleles” — or gene variants — “you will get the disease.” Most people with these deterministic genes start to develop early-onset Alzheimer’s, starting in their 60s, or even their 40s and 50s, Lyketsos says. But they are rare, accounting for only around 5% of all cases of Alzheimer’s. “Everybody else has what we call a sporadic [genetic] risk,” says Lyketsos.

Outside of these rare genes, there are more than 100 genes as well as environmental and behavioral factors that contribute to a person’s Alzheimer’s risk profile. The No. 1 risk factor is age; the odds of developing the disease double about every five years after age 65, according to the National Institute on Aging. Rowlands is now 94, so she would have been around 89 at the time of her diagnosis. “People who develop Alzheimer’s at this age and had a parent who had it are at increased risk, but it’s more of a sporadic type of risk,” Lyketsos says.

Another important risk factor is cerebrovascular disease, in which cerebral blood vessels and blood flow are compromised. High blood pressure and smoking are both risk factors for brain vascular disease, which, in turn, increases risks for Alzheimer’s disease, “but there aren’t any clear environmental causes that we can say for sure lead to Alzheimer’s,” says Lyketsos.

What to do if you have a family member with Alzheimer’s

While Alzheimer’s does run in families, “we don’t advise genetic testing right now in people who are asymptomatic” in most cases, Lyketsos says. “But if there is a strong family pedigree of disease, and of somebody getting it at a young age, we will test.”

Having a parent or other close relative with Alzheimer’s can lead to some anxiety, adds Lyketsos. “Not infrequently, I will have a child of someone who developed dementia or Alzheimer’s … come in concerned,” he says. “But most of the time, in that situation, we can either not measure the memory loss, or there are other explanations, like poor sleep, consumption of alcohol or depression, that might be causing changes that mimic dementia or mild cognitive impairment” and may be unrelated to Alzheimer’s or their loved one’s diagnosis, Lyketsos says.

Lyketsos says that there are no special precautions to take if you have a family history of Alzheimer’s, but there are many generally pro-health habits that are particularly important if you know that you’re at risk, including staying active both mentally and physically, eating a balanced diet and maintaining overall brain-and-body health, he says. Beyond that, Lyketsos advises getting a combination of aerobic and anaerobic exercise “that you do regularly, so pick something you will do.”

And last but not least: “This is a very important thing: Be socially engaged,” says Lyketsos. “Loneliness seems to be a risk factor.” Be especially sure to stay engaged and get social support if you’re caring for a loved one with Alzheimer’s, he adds. “Get in touch with professionals who can help guide you in setting up a safe, stable environment, but also who can help you take care of yourself, because it’s a very long road.”