Alzheimer’s disease is a progressive brain disorder that destroys memory, thinking skills, and eventually the ability to carry out basic daily tasks. It is the most common cause of dementia, and an estimated 7.4 million Americans age 65 and older are living with it. Deaths from Alzheimer’s increased 134% between 2000 and 2024, making it the fifth-leading cause of death among people 65 and older.
What Happens Inside the Brain
Two abnormal proteins drive the disease. The first, called amyloid-beta, normally exists in the brain and may serve a protective role, possibly as a first responder to injury. But when the brain produces too much of it and its cleanup systems can’t keep pace, amyloid-beta clumps together into sticky deposits called plaques that accumulate between nerve cells.
The second protein, tau, has a useful job under normal conditions: it acts like internal scaffolding, stabilizing the tiny structural tubes inside each nerve cell so they can transport nutrients and signals. In Alzheimer’s, certain chemical changes cause tau molecules to bunch together into tangles inside the nerve cells themselves. While amyloid plaques pile up between cells, these tangles form within them and are thought to be directly lethal to the neurons they were meant to protect.
The hippocampus, a seahorse-shaped region deep in the brain responsible for forming new memories, is among the first areas hit. Research from Weill Cornell Medicine shows that specific subregions of the hippocampus can shrink by 10 to 25 percent even before a person shows obvious symptoms. As the disease advances, this shrinkage spreads to neighboring areas, and the damage fans outward to regions governing language, reasoning, and eventually basic bodily functions like swallowing and walking.
Early Signs vs. Normal Aging
Everyone forgets things occasionally. Misplacing your keys, blanking on someone’s name, or missing a payment once in a while are all part of normal aging. What separates Alzheimer’s from ordinary forgetfulness is a pattern of decline that disrupts daily life. The National Institute on Aging draws a useful contrast:
- Normal aging: forgetting which day it is and remembering later. Alzheimer’s: losing track of the date, season, or time of year.
- Normal aging: sometimes searching for the right word. Alzheimer’s: trouble holding a conversation.
- Normal aging: making a bad decision once in a while. Alzheimer’s: consistently poor judgment and decision-making.
- Normal aging: losing things from time to time. Alzheimer’s: frequently misplacing items and being unable to retrace steps to find them.
Other warning signs include asking the same question repeatedly, getting lost in familiar places, difficulty following recipes or directions, growing confusion about people and places, and neglecting personal care such as bathing or eating properly.
How the Disease Progresses
Alzheimer’s unfolds over years, often decades, in a roughly predictable sequence. The earliest stage is entirely silent. Called preclinical Alzheimer’s, it involves measurable brain changes, like amyloid buildup, but produces no noticeable symptoms. This phase can last 15 to 20 years before anyone suspects a problem.
The next stage, mild cognitive impairment, brings subtle but real changes. You might have memory lapses for information that would normally come easily: recent conversations, upcoming appointments, recent events. Judging how long a task will take or figuring out the steps to complete it becomes harder. These changes are noticeable but not severe enough to interfere with work or relationships.
Mild dementia is typically when Alzheimer’s is formally diagnosed, because the problems become obvious to family and friends. Recent memories slip away more consistently. Handling finances, planning meals, or navigating new situations grows difficult. Personality changes may appear: increased anxiety, withdrawal from social activities, or uncharacteristic irritability.
In moderate dementia, damage spreads beyond memory centers. People may struggle to recognize family members, wander and get lost, have difficulty dressing or bathing, and experience significant personality and behavioral changes including agitation, suspicion, or sleep disruption. This stage typically requires increasing daily assistance and is often the longest phase of the illness.
Severe dementia, the final stage, involves near-total dependence. Communication dwindles to a few words or none at all. The ability to walk, sit upright, and swallow gradually disappears. Round-the-clock care becomes necessary.
Risk Factors and Genetics
Age is the strongest risk factor. Most people diagnosed are 65 or older, and the likelihood roughly doubles every five years after that. But Alzheimer’s is not a normal part of aging.
Genetics play a significant role. The gene most strongly linked to the common, late-onset form of the disease is called APOE. Everyone carries two copies, and the e4 variant substantially raises risk. Having one copy of APOE-e4 doubles or triples your likelihood of developing Alzheimer’s. Carrying two copies increases the risk 8 to 12 times. Still, many people with APOE-e4 never develop the disease, and many people without it do.
A 2024 report from the Lancet Commission identified 14 modifiable risk factors that collectively account for a large share of dementia cases worldwide. These include lower educational attainment, hearing loss, high blood pressure, smoking, obesity, depression, physical inactivity, diabetes, excessive alcohol consumption, traumatic brain injury, and social isolation, among others. Addressing these factors, particularly in midlife, can meaningfully reduce or delay the onset of dementia even in people with genetic risk.
How Alzheimer’s Is Diagnosed
Diagnosis starts with a thorough clinical evaluation: medical history, cognitive testing, neurological exams, and ruling out other conditions that can mimic dementia (thyroid disorders, vitamin deficiencies, depression, medication side effects). Brain imaging with MRI can reveal hippocampal shrinkage and rule out strokes or tumors.
When the cause of cognitive decline remains uncertain after that workup, specialized tests can detect Alzheimer’s-specific brain changes. Amyloid PET scans image the plaques directly, and tau PET scans map the tangle buildup. Spinal fluid analysis can measure the ratio of key proteins that correlate closely with what PET scans show.
A major recent development is a blood test. In early 2025, the FDA cleared the first blood-based diagnostic for Alzheimer’s, which measures a ratio of two proteins in a standard blood draw. In clinical studies of 499 cognitively impaired adults, 91.7% of people who tested positive actually had amyloid plaques confirmed by PET scan or spinal fluid testing. Among those who tested negative, 97.3% were truly negative. This test doesn’t replace a full clinical evaluation, but it can help doctors identify who needs further workup and avoid unnecessary invasive procedures for those who don’t.
Treatment Options
For decades, Alzheimer’s treatments could only manage symptoms. Medications that boost certain brain chemicals can temporarily improve memory and thinking in some people, though they don’t slow the underlying disease.
That changed with the approval of a new class of drugs that target amyloid plaques directly. The FDA has approved two antibody-based infusion treatments: lecanemab (Leqembi) and donanemab (Kisunla). These drugs are given by IV and work by binding to amyloid plaques and helping the brain clear them. In a clinical trial of 1,736 patients with early-stage Alzheimer’s, donanemab significantly slowed the rate of cognitive and functional decline compared to placebo over 76 weeks. These treatments don’t stop or reverse the disease, but they represent the first therapies proven to slow its progression.
Eligibility requires biomarker confirmation of amyloid in the brain, typically through PET scan, spinal fluid testing, or the newer blood test. The treatments carry risks, most notably brain swelling or small brain bleeds that are usually mild but require monitoring with regular MRI scans. People who carry two copies of the APOE-e4 gene variant face higher risk of these side effects.
The Cost of Care
Alzheimer’s exacts an enormous financial toll. In 2026, health and long-term care costs for people living with Alzheimer’s and other dementias are projected to reach $409 billion in the United States alone, and that figure excludes the value of unpaid caregiving provided by family members. Most of that cost falls on nursing home and home health care rather than medications or hospital stays, reflecting the years of hands-on daily support the disease demands. The typical person with Alzheimer’s lives four to eight years after diagnosis, though some live as long as 20, and the level of care required escalates steadily throughout.