Dementia develops when cells in the parts of your brain involved with mental ability become damaged. Damage to these cells can be caused by:

• diseases and infections that affect the brain, such as Alzheimer's disease or meningitis,
• pressure on the brain, for example from a brain tumor,
• lack of blood and oxygen supply to the brain, for example due to stroke, and
• head injury.

The most common type of dementia is that caused by Alzheimer's disease, which is responsible for about 60% of all cases. The cause of Alzheimer's disease is unknown, except for a very small percentage of cases that are inherited.

About 20% of all cases of dementia are vascular dementias, the name given to dementia caused by poor circulation of blood in the brain. There are a number of different types of vascular dementia, the most common being multi-infarct dementia, where the small blood vessels in the brain becoming narrowed or blocked. The cells in the brain supplied by these blood vessels become starved of the blood and oxygen they need, and become damaged or die. This is like having a series of small strokes in the brain.

The risk of developing vascular dementia is increased by the same things that increase the risk of stroke and heart disease. These include high blood pressure, diabetes, smoking, poor diet and excessive alcohol intake.

Other less common types of dementia include:

• Lewy body dementia,
• dementia caused by Picks disease,
• dementia caused by Huntington's disease,
• dementia caused by Creutzfeld-Jakob disease (CJD),
• dementia caused by the advanced stages of syphilis or HIV/AIDS,
• dementia caused by hypothyroidism (under-activity of the thyroid gland),
• dementia caused by build-up of fluid within the skull leading to pressure on the brain (hydrocephalus),
• dementia caused by infections of the brain, such as meningitis or encephalitis,
• dementia as part of a neurological (brain) illness, such as Parkinson's disease or multiple sclerosis,
• dementia caused by reaction to a medication, or to an interaction of two or more medications,
• dementia caused by long-term alcohol misuse,
• dementia caused by use of illegal drugs, and
• dementia caused by lack of certain hormones or nutrients, especially B vitamins.