June 28, 2016 2:43 PM EDT

Alzheimer’s disease, the most common form of dementia, is usually a disease of the elderly. Symptoms generally first start to appear when people are in their mid-60s. Rarely, Alzheimer’s also affects people much younger, sometimes people in their 40s and 50s. It struck Pat Summitt, the renowned basketball coach who captured the most wins ever in Division 1 basketball, at age 59. Just five years later, at 64, early-onset Alzheimer’s claimed her life.

Early-onset Alzheimer’s accounts for 5% of the nearly 5 million Alzheimer’s cases in the U.S. Alzheimer’s is the 6th leading cause of death in the U.S.

Here’s what else you need to know.

What is early-onset Alzheimer’s disease?

“It’s a particularly tragic form of Alzheimer’s disease, because it typically affects people during their peak time of productivity,” says Dr. Thomas Wisniewski, director of the Center for Cognitive Neurology at NYU Langone Medical Center. Early Alzheimer’s disease is thought to have a greater genetic influence than the kind that strikes later, and “much more commonly, there’s a family history,” Wisniewski says.

What causes it?

Why some people get Alzheimer’s disease is still mostly a mystery. But most people who get early-onset Alzheimer’s disease have a family history, Wisniewski says, and about 10% of cases are caused by mutations in three known genes: presenilin 1, presenilin 2 and amyloid beta precursor protein (APP). These mutations are closely linked to the overproduction of amyloid beta, sticky proteins that can clump together form plaques in the brain, suffocating neurons. As for the cause of the other 90% of early Alzheimer’s cases: “We just don’t know,” Wisniewski says. “A lot more work has to be done on this.”

Excessive amyloid seems to play a key role in early-onset Alzheimer’s. The largest set of people who have the disease are those with Down syndrome—people born with three copies of chromosome 21. “That chromosome is where the amyloid precursor protein is, so they produce a lot more amyloid beta,” Wisniewski says, adding that nearly all people with Down syndrome show Alzheimer’s disease pathology.

What are the symptoms?

Short-term memory loss is a symptom in the majority of Alzheimer’s cases, and while that’s still true of early-onset, this rare form sometimes presents in ways that have nothing to do with memory. In about a quarter of cases, people with early-onset have difficulty controlling their gaze, processing visual information, doing calculations or have problems with speech, says Wisniewski.

How long do people usually live after diagnosis?

Otherwise healthy people will typically live for another 10 to 12 years, Wisniewski says. “The pathology of early Alzheimer’s is more severe, so that can lead to a shorter survival, but a lot is dependent on how much nursing and supportive care they get.”

How do people actually die from Alzheimer’s?

Alzheimer’s can cause medical complications, like infections, that are driven by the disease. “People will most commonly die of aspiration pneumonia; when they’re trying to take in food or liquids, it just goes to their lungs and they die,” Wisniewski says. As Alzheimer’s progresses, people can develop compromised immune systems, making it harder to fight off infection and be physically active. Because it complicates all other medical conditions, and because it requires such comprehensive care, Alzheimer’s disease is the single most costly condition in America.

More Must-Read Stories From TIME

Write to Mandy Oaklander at mandy.oaklander@time.com.

Read More From TIME
You May Also Like