Which illnesses cause dementia?

When doctors think that a person may have dementia, they try to work out which illness is causing the problems.

These illnesses are not like chicken pox or flu. They don’t spread from person to person, so you can’t ‘catch’ dementia from someone else.

This chart tells you about the four main illnesses that can affect a person’s brain and cause dementia.

Each illness can affect a different part of the brain at first. Some people might have problems with their memory or behaviour to begin with. Others might have problems speaking or seeing what or where things are. As time goes on, these illnesses can affect more of a person’s brain and cause more problems.


Alzheimer’s disease

Alzheimer’s is a disease that causes dementia. It causes about half of all cases of dementia. When people have this disease, their brain starts to change. No one fully understands what causes a person’s brain to start changing in this way. The changes usually happen slowly at first, but over time they can have a big effect on how the brain works.

Vascular dementia

This is the second most common cause of dementia. It happens when less blood reaches the brain than normal. Brain cells are damaged because they don’t get enough food and oxygen, and this causes dementia. Sometimes this happens very suddenly, when a person has a stroke. Sometimes, the damage happens slowly over time.

Dementia with Lewy bodies

Dementia with Lewy bodies is sometimes called DLB for short. It’s the third most common cause of dementia. Lewy bodies are tiny, round clumps of protein that build up inside brain cells. They stop brain cells from working properly, causing dementia. They can cause other problems too, such as hallucinations (seeing things that aren’t really there), slowed, stiff arm and leg movements and changes to sleep, causing people to talk or move about in their sleep. People with DLB often have big differences between their good days and bad days. On bad days, they may be much more confused and sleepy.

Frontotemporal dementia

Frontotemporal dementia is sometimes called FTD. It’s caused by damage to the parts of the brain that control personality, emotions, behaviour, thinking and language. This can cause different problems, depending on which parts of the brain are most affected.


This information was updated in November 2019 and is due for review in November 2021. Please contact us if you would like a version with references.