Alzheimer's disease is caused by parts of the brain shrinking (atrophy), which affects the structure and function of particular brain areas.
It's not known exactly what causes this process to begin. However, in the brains of people with Alzheimer's disease, scientists have found amyloid plaques (abnormal deposits of protein), neurofibrillary tangles (containing tau) and imbalances in a chemical called acetylcholine.
It's also common to have a degree of vascular damage 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. The first areas affected are responsible for memories.
Although it's still unknown what triggers Alzheimer's disease, several factors are known to increase your risk of developing the condition.
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's 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 the age of 40.
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 the genetics of dementia.
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.
People who have had a severe head injury have been found to be at higher risk of developing Alzheimer's disease.
Research shows that several lifestyle factors and conditions associated with cardiovascular disease can increase the risk of Alzheimer's disease.
You can help reduce your risk by:
Read more about reducing your risk of Alzheimer's disease.