Alzheimer’s disease is a progressive brain disorder that slowly destroys memory, thinking skills, and eventually the ability to carry out basic daily tasks. It is the most common cause of dementia, accounting for 60 to 80 percent of all dementia cases. An estimated 7.2 million Americans age 65 and older are living with Alzheimer’s dementia in 2025, and that number could reach 13.8 million by 2060.
Alzheimer’s is not a normal part of aging. While occasional forgetfulness is common as people get older, Alzheimer’s involves a steady decline in brain function that worsens over time and ultimately affects every aspect of a person’s life.
How Alzheimer’s Differs From Dementia
Dementia is not a single disease. It’s an umbrella term for a group of symptoms, including memory loss, confusion, and difficulty with reasoning, that are severe enough to interfere with daily life. Alzheimer’s is one specific disease that causes dementia. Other conditions can also cause dementia, including Lewy body disease, vascular disease, and frontotemporal degeneration. When someone is diagnosed with dementia, the next step is figuring out which disease is behind it.
What Happens in the Brain
Two types of abnormal protein buildup drive Alzheimer’s. The first involves a protein fragment called beta-amyloid, which clumps together into sticky plaques between brain cells. In a healthy brain, these fragments are broken down and cleared away. In Alzheimer’s, they accumulate into dense deposits that disrupt communication between neurons.
The second involves a protein called tau, which normally helps stabilize the internal structure of brain cells. In Alzheimer’s, tau becomes abnormally modified: a healthy tau molecule carries about two chemical tags (phosphate groups), but in an Alzheimer’s brain, tau molecules carry around eight. This overloaded tau collapses into tangled fibers inside neurons, eventually killing the cells from within.
Together, these amyloid plaques and tau tangles interfere with a process called long-term potentiation, which is how the brain strengthens connections to form and store memories. As more neurons are damaged and die, entire brain regions shrink, starting with areas responsible for memory and gradually spreading to regions that control language, reasoning, and physical movement.
Early Warning Signs
The CDC lists 10 warning signs that distinguish Alzheimer’s from typical age-related changes:
- Memory loss that disrupts daily life
- Challenges in planning or solving problems
- Difficulty completing familiar tasks at home, work, or during hobbies
- Confusion about time or place
- Trouble understanding visual images and spatial relationships
- New problems with words in speaking or writing
- Misplacing things and being unable to retrace steps
- Decreased or poor judgment
- Withdrawal from work or social activities
- Changes in mood or personality
The key distinction is severity and pattern. Forgetting where you left your keys is normal. Forgetting what keys are for, or placing them somewhere unusual (like the refrigerator) and having no memory of doing so, is not.
How Symptoms Progress Over Time
Alzheimer’s typically moves through three broad stages, though the pace varies widely from person to person.
Mild Stage
People in the early stage can still work and socialize, but they start having trouble with things that used to come easily. They may forget recent conversations or appointments, struggle to judge how much time a task will take, or have difficulty organizing the steps to complete something. Decision-making becomes harder. At this point, friends and family often notice something is off before the person does.
Moderate Stage
This is usually the longest stage and the one that demands the most from caregivers. Confusion deepens. Some people become suspicious of those around them, sometimes accusing a spouse of infidelity or believing someone is stealing from them. Others may see or hear things that aren’t there. Restlessness and agitation are common, especially late in the day, a pattern sometimes called “sundowning.” Some people develop outbursts of aggressive behavior. Help with dressing, bathing, and other daily routines becomes necessary.
Severe Stage
In the final stage, mental function continues to decline and physical capabilities deteriorate significantly. People generally lose the ability to communicate, to control movement, and eventually to swallow. Round-the-clock care is required. The body becomes vulnerable to infections, particularly pneumonia, which is a common cause of death in late-stage Alzheimer’s.
Risk Factors You Can and Can’t Control
Age is the biggest risk factor. Most people with Alzheimer’s are 65 or older. Genetics also play a role. One of the strongest genetic risk factors (in people of European descent) is a variant of the APOE gene called APOE4. People who inherit two copies of this variant, about 2 percent of the general population, have roughly a 60 percent chance of developing Alzheimer’s dementia by age 85. Despite being such a small slice of the population, they account for an estimated 15 percent of all Alzheimer’s cases.
But genetics aren’t destiny. A 2024 update from The Lancet Commission identified 14 modifiable risk factors for dementia. These include less education, hearing loss, high blood pressure, smoking, obesity, depression, physical inactivity, diabetes, excessive alcohol consumption, traumatic brain injury, air pollution, social isolation, untreated vision loss, and high LDL cholesterol. Addressing these factors across a lifetime can meaningfully reduce your risk. Reducing vascular damage through measures like quitting smoking and treating high blood pressure has already contributed to a decline in age-related dementia rates in some populations.
How Alzheimer’s Is Diagnosed
There is no single test for Alzheimer’s. Diagnosis typically involves a combination of cognitive assessments, medical history review, and increasingly, biological testing. Brain imaging with PET scans can detect amyloid plaques in a living person’s brain, providing direct evidence of Alzheimer’s pathology. Spinal fluid tests can measure levels of specific proteins: low levels of one form of amyloid and high levels of tau in the fluid surrounding the brain and spinal cord are a strong signal of the disease. These biomarker tests are especially valuable for catching Alzheimer’s early, sometimes before significant symptoms appear.
Current Treatments
No treatment cures Alzheimer’s, but two categories of medication can help manage symptoms. The first group, cholinesterase inhibitors (including donepezil, rivastigmine, and galantamine), works by boosting levels of a chemical messenger involved in memory and learning. These are typically used in the mild to moderate stages. The second is memantine, which works differently: it blocks excessive signaling from a brain chemical called glutamate that, when overactive, damages and kills neurons. Memantine is often used in combination with a cholinesterase inhibitor for moderate to severe Alzheimer’s.
A newer class of treatment has recently reached the market. These are antibody-based therapies designed to clear amyloid plaques from the brain rather than just manage symptoms. Lecanemab received full FDA approval and works by targeting and removing amyloid buildup. Donanemab uses a similar approach. Both have shown the ability to slow cognitive decline in people with early-stage Alzheimer’s, though the degree of benefit and the risk of side effects, particularly brain swelling and small brain bleeds, are still being closely evaluated in real-world use. These drugs are not effective in later stages of the disease, making early detection increasingly important.