Alzheimer's disease is caused by parts of the brain wasting away (atrophy), which damages the structure of the brain and how it works.

It is not known exactly what causes this process to begin, although people with Alzheimer's disease have been found to have abnormal amounts of protein (amyloid plaques), fibres (tau tangles) and a chemical called acetylcholine in the brain.

These reduce the effectiveness of healthy neurons (nerve cells that carry messages to and from the brain), gradually destroying them.

Over time, this damage spreads to several areas of the brain, such as the grey matter (responsible for processing thoughts) and the hippocampus (responsible for memory).

Increased risk

Although it is still unknown what triggers Alzheimer's disease, several factors are known to increase your risk of developing the condition.

Age

Age is the single most significant factor in the development of Alzheimer's disease. The likelihood of developing the condition doubles every five years after you reach 65 years of age.

However, it is not just older people who are at risk of developing Alzheimer's disease. Around 1 in 20 people with the condition are under 65. This is called early-onset Alzheimer's disease and it can affect people from around 40 years of age.

Family history

The genes you inherit from your parents can contribute to your risk of developing Alzheimer's disease, although the actual increase in risk is small if you have a close family member with the condition.

However, in a few families, Alzheimer's disease is caused by the inheritance of a single gene, and the risks of the condition being passed on are much higher.

If several of your family members have developed dementia over the generations, it may be appropriate to seek genetic counselling for information and advice about your chances of developing Alzheimer's disease when you are older.

The Alzheimer's Society website has more information about genetics and dementia.

Down's syndrome

People with Down's syndrome are at a higher risk of developing Alzheimer's disease.

This is because the genetic fault that causes Down's syndrome can also cause amyloid plaques to build up in the brain over time, which can lead to Alzheimer's disease in some people.

Whiplash and head injuries

People who have had a severe head injury or severe whiplash (a neck injury caused by a sudden movement of the head) have been found to be at higher risk of developing Alzheimer's disease.

Cardiovascular disease

Research shows that several lifestyle factors and conditions associated with cardiovascular disease can increase the risk of Alzheimer's disease.

These include:

• smoking
• obesity 
• diabetes 
• high blood pressure
• high cholesterol

You can help reduce your risk by stopping smoking if you smoke, eating a healthy, balanced diet, and having regular health checks as you get older. It is also important to keep as active as possible, both mentally and physically, to help reduce the risk of Alzheimer's disease.

Read more about reducing your risk of Alzheimer's disease.