Memory Digest

What Is Dementia?

Dementia is an umbrella term for a decline in memory and thinking that interferes with daily life. Here's what it is, what causes it, and what it isn't.

Last reviewed: October 2026

Dementia is not one disease. It is a general term—an umbrella—for a loss of memory, thinking, language, problem-solving and other abilities that is serious enough to interfere with daily life. Many different diseases can cause it, much as many different conditions can cause heart failure.

How common is it?

The World Health Organization estimates that more than 55 million people worldwide live with dementia, with nearly 10 million new cases each year. In the United States, the Alzheimer's Association estimates that more than 7 million people aged 65 and older are living with Alzheimer's dementia alone. Risk rises steeply with age, but dementia is not a normal part of aging—most older people never develop it.

What are the symptoms?

Symptoms depend on the cause and on which parts of the brain are affected, but often include:

• Memory loss, especially for recent events
• Trouble finding words or following conversations
• Difficulty planning, organizing or solving problems
• Getting confused about time or place, or lost in familiar areas
• Problems with judgment, such as falling for scams
• Changes in mood, personality or behavior—anxiety, suspicion, apathy, irritability
• Difficulty with visual and spatial tasks, like judging distances

See 10 Early Warning Signs of Alzheimer's and Dementia for early signs to watch for.

What causes dementia?

Dementia happens when nerve cells in the brain stop working, lose connections with other cells, and die. The most common causes are:

• Alzheimer's Disease – the most common cause, accounting for an estimated 60–80% of cases. Abnormal amyloid plaques and tau tangles build up in the brain.
• Vascular Dementia – caused by reduced blood flow to the brain from strokes or damage to small blood vessels.
• Lewy Body Dementia – caused by clumps of the protein alpha-synuclein; often includes visual hallucinations, fluctuating alertness and movement problems.
• Frontotemporal Dementia – damage to the front and side lobes, often changing personality, behavior or language first; tends to start younger.
• Mixed dementia – more than one cause at the same time, which is very common in older adults, especially Alzheimer's plus vascular changes.

Less common causes include Parkinson's disease dementia, Creutzfeldt-Jakob disease, Huntington's disease, and brain changes after severe or repeated head injuries.

Conditions that look like dementia

Some conditions cause memory problems that can be treated or reversed, which is why a proper medical evaluation matters. These include medication side effects, vitamin B12 deficiency, thyroid problems, depression, sleep apnea, normal pressure hydrocephalus, and infections. Sudden confusion is more likely to be delirium, which is a medical emergency. See Reversible Causes of Memory Problems and Delirium vs. Dementia.

Mild Cognitive Impairment (MCI) (MCI) is a stage of noticeable thinking problems that do not yet interfere significantly with independence. It sometimes, but not always, progresses to dementia.

Is dementia hereditary?

Most dementia is not directly inherited. Having a parent or sibling with Alzheimer's raises your risk somewhat, and some genes—especially APOE4—increase risk without guaranteeing disease. Rare inherited forms, caused by single gene changes, usually appear before age 65. A genetic counselor can help families with a strong history decide whether testing makes sense.

Can dementia be treated?

Most causes of dementia cannot yet be cured, but there is real help:

• Medicines can ease symptoms for some people, and new anti-amyloid drugs can modestly slow early Alzheimer's (see Dementia Treatments: An Overview).
• Treating blood pressure, hearing loss, depression and sleep problems can help thinking and quality of life.
• Non-drug approaches—routine, exercise, social activity, music, and support for caregivers—make a big difference day to day.
• Planning ahead for legal, financial and care decisions gives people a voice in their future.

Can it be prevented?

There is no guaranteed way to prevent dementia, but the 2024 Lancet Commission estimated that nearly half of cases worldwide might be linked to 14 modifiable risk factors, including high blood pressure, hearing loss, physical inactivity, smoking and social isolation. See Reducing Dementia Risk: The 14 Risk Factors.

What to do if you're worried

If you notice changes in yourself or someone you love, talk to a doctor. An early, accurate diagnosis can rule out treatable causes, open access to treatments and clinical trials, and give time to plan. See Getting a Diagnosis. For free, confidential support any time, call the Alzheimer's Association 24/7 Helpline at 800-272-3900.

Sources

1. National Institute on Aging: What Is Dementia?
2. Alzheimer's Association: What Is Dementia?
3. World Health Organization: Dementia fact sheet
4. CDC: About Dementia

Not medical advice. This article is general information, not a diagnosis or treatment plan for any individual. Talk with a doctor about your situation.

Related articles

• Alzheimer's Disease
• 10 Early Warning Signs of Alzheimer's and Dementia
• Getting a Diagnosis
• Stages of Dementia
• Delirium vs. Dementia