Alzheimer’s disease is a specific brain disease that progressively destroys memory, thinking ability, and eventually the capacity to carry out basic daily tasks. It is the most common cause of dementia, accounting for the majority of cases, and currently affects more than 7 million Americans age 65 and older. By 2050, that number is projected to reach nearly 13 million.
A common point of confusion: dementia is not a disease itself. It’s an umbrella term describing a wide range of symptoms, including memory loss, impaired reasoning, and personality changes. Alzheimer’s is one specific disease that causes dementia, but other conditions like vascular disease, Lewy body disease, and frontotemporal degeneration can cause dementia too.
What Happens in the Brain
Alzheimer’s is defined by two abnormalities that develop in brain tissue: amyloid plaques and neurofibrillary tangles.
Amyloid plaques are unusual clumps of a protein called beta amyloid that build up in the spaces between nerve cells, along with degenerating bits of neurons. Neurofibrillary tangles form inside the neurons themselves. They’re made of a protein called tau, which normally helps maintain the tiny transport highways that carry nutrients and signals within each nerve cell. In Alzheimer’s, tau becomes misshapen and twists into tangled bundles. When that happens, the cell’s internal transport system collapses. Nerve cells can no longer communicate properly, and they begin to die.
Scientists still aren’t certain whether plaques and tangles directly cause the damage or whether they’re byproducts of some deeper process that kills neurons. What is clear is that both increase as the disease progresses, and the pattern of their spread through the brain closely tracks the symptoms a person develops.
A Biological Disease, Not Just Symptoms
The medical understanding of Alzheimer’s has shifted significantly in recent years. Updated criteria from 2024 define Alzheimer’s as a biological disease, meaning it’s identified by measurable changes in the brain rather than by symptoms alone. This matters because brain changes can begin 15 to 20 years before a person notices any memory problems.
A research framework from the National Institute on Aging organizes Alzheimer’s biology into three categories: beta-amyloid deposits (A), abnormal tau (T), and neurodegeneration or nerve cell injury (N). Under this system, only people who show both amyloid and tau abnormalities are considered to have Alzheimer’s disease. This “A/T/N” framework places the disease on a continuum, from the earliest silent brain changes through severe dementia.
For now, this biological approach is mainly used in research settings. In everyday clinical practice, doctors still rely heavily on cognitive testing, medical history, and imaging to evaluate people who already have symptoms.
How Alzheimer’s Progresses
The disease moves through three broad stages, each defined by how much it affects daily life.
Preclinical stage. Brain changes are underway, but there are no noticeable symptoms. Neither the person nor their family can tell anything is wrong. This stage is currently identified only in research settings.
Mild cognitive impairment (MCI). Mild changes in memory and thinking become apparent. A person might struggle more with finding words, keeping track of conversations, or remembering recent events. These changes are real but not severe enough to interfere with work or relationships.
Dementia stage. This is when most people receive a formal diagnosis. Memory and thinking problems become significant enough to disrupt daily functioning. The dementia stage itself ranges from mild (needing help with some tasks like managing finances) to severe (requiring full-time care for basic needs like eating and bathing). Alzheimer’s is the fifth-leading cause of death among Americans 65 and older, and one in three older Americans dies with Alzheimer’s or another dementia.
How It’s Diagnosed
For most of Alzheimer’s history, a definitive diagnosis was only possible after death, through autopsy. That has changed. Doctors can now detect the hallmark proteins in living people using brain imaging scans and cerebrospinal fluid tests. PET scans use special tracers that bind to amyloid plaques or tau tangles, making them visible. Spinal fluid analysis can measure levels of amyloid and tau proteins directly.
In 2025, the FDA cleared the first blood test for Alzheimer’s diagnosis. The test measures the ratio of two proteins (a form of tau and a fragment of amyloid) in a standard blood draw. In clinical studies, 91.7% of people who tested positive on the blood test were confirmed to have amyloid plaques by PET scan or spinal fluid testing. Among those who tested negative, 97.3% were confirmed negative. Less than 20% of patients received an unclear result. This is a major step toward making diagnosis faster and more accessible, since blood tests are far simpler and cheaper than PET scans or spinal taps.
Genetics and Risk
Alzheimer’s comes in two forms with different genetic profiles. The vast majority of cases are late-onset, appearing after age 65. The most significant genetic risk factor for this form is a gene variant called APOE e4. Carrying one copy of APOE e4 doubles or triples the risk of developing Alzheimer’s. Carrying two copies increases risk 8 to 12 times. But APOE e4 is a risk factor, not a guarantee. Many people with the variant never develop the disease, and many people without it do.
A much rarer form, called young-onset or familial Alzheimer’s, has a direct genetic cause. Scientists have identified three gene mutations (in genes called APP, PSEN1, and PSEN2) that virtually guarantee a person will develop symptoms before age 65. Inheriting just one copy of any of these mutations from either parent is enough. This form accounts for a small fraction of all cases but tends to progress more aggressively.
The Scale of the Problem
Alzheimer’s is not just a medical crisis. It’s a financial one. Projected health and long-term care costs for people living with Alzheimer’s and other dementias will reach $409 billion in 2026, and that figure doesn’t include the value of unpaid caregiving provided by family members. By 2050, total costs are expected to approach $1 trillion. Deaths from Alzheimer’s increased 134% between 2000 and 2024, driven largely by an aging population and better recognition of the disease as a cause of death.