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Dementia guide

What is dementia?

An umbrella term for many diseases that affect memory, thinking and behavior — and some look-alikes that can be treated.

Dementia is not one disease. It's a general term for a loss of memory, thinking, language, judgment or behavior that is serious enough to interfere with daily life. Many different diseases can cause it. Dementia is not a normal part of aging — most older people never develop it.

The main types

Alzheimer's disease

Most common · 60–80%

Abnormal proteins (amyloid plaques and tau tangles) build up and damage brain cells. It usually begins with trouble remembering new information, then slowly affects language, reasoning and daily tasks.

Vascular dementia

Blood vessels

Caused by reduced blood flow — after a stroke or from many small blood-vessel changes. It often affects planning, speed of thinking and attention more than memory at first, and may worsen in steps.

Lewy body dementia

Includes Parkinson's dementia

Thinking that changes a lot from day to day, vivid visual hallucinations, acting out dreams in sleep, and Parkinson-like stiffness or slowness. People with Lewy body dementia can react badly to some antipsychotic medicines — always tell doctors about the diagnosis.

Frontotemporal dementia (FTD)

Often starts 45–65

Affects the front and sides of the brain. Early signs are often changes in personality and behavior — loss of tact or empathy, impulsive actions, compulsive habits — or problems with language, while memory may stay fairly good at first.

Mixed dementia

Very common in the oldest

More than one cause at once — most often Alzheimer's plus blood-vessel changes.

Mild cognitive impairment (MCI)

MCI means memory or thinking has changed more than expected for a person's age, but they still manage daily life on their own. Some people with MCI go on to develop dementia; others stay stable or even improve — especially when a treatable cause is found. MCI is a reason for follow-up, not a reason to panic.

How dementia usually progresses

Early (mild)Mostly independent. Forgets recent events, repeats questions, has trouble with bills, planning or finding words. Often the person notices changes too.
Middle (moderate)Often the longest stage. Needs help with daily tasks, may get lost, confuse people, and have mood or behavior changes and sleep problems.
Late (severe)Needs full-time care. Speech becomes limited; walking, eating and swallowing become difficult. Comfort and connection matter most.

Every person's path is different. Some people live many years in the early stage.

Conditions that can look like dementia — and can be treated

This is why a proper medical evaluation matters. Memory and thinking problems can be caused or worsened by:

  • Medicine side effects or interactions — especially sleep aids, some bladder, allergy and anxiety medicines
  • Depression (see dementia, depression, or delirium?)
  • Low vitamin B12 or thyroid problems
  • Infections, dehydration, or kidney and liver problems
  • Untreated hearing or vision loss
  • Sleep apnea and chronic poor sleep
  • Heavy alcohol use
  • Normal pressure hydrocephalus — extra fluid in the brain, often with walking problems and bladder control problems

Sources

Reviewed September 2026. Education only — not medical, legal or financial advice.