Dementia is a term used to describe various symptoms of cognitive decline such as forgetfulness, but is not a clinical diagnosis itself until an underlying disease or disorder has been identified.
Dementia is not a single disease in itself, but a general term to describe symptoms such as impairments to memory, communication and thinking. Alzheimer's disease is the best known and most common disorder under the umbrella of dementia.
While the likelihood of having dementia increases with age, it is not a normal part of aging. Before we had today's understanding of specific disorders, "going senile" used to be a common phrase for dementia ("senility"), which misunderstood it as a standard part of getting old.
Light cognitive impairments, by contrast, such as poorer short-term memory, can happen as a normal part of aging (we slowly start to lose brain cells as we age beyond our 20s). This is known as age-related cognitive decline, not dementia, because it does not cause the person or the people around them any problems.
Dementia describes two or more types of symptom that are severe enough to affect daily activities.
What causes dementia?
All dementias are caused by brain cell death, and neurodegenerative disease - progressive brain cell death that happens over a course of time - is behind most dementias.
But as well as progressive brain cell death like that seen in Alzheimer's disease, dementia can be caused by a head injury, a stroke or a brain tumor, among other causes.
Some types of traumatic brain injury - particularly if repetitive, such as received by sports players - have been linked to certain dementias appearing later in life. Evidence is weak, however, that a single brain injury will raise the likelihood of having a degenerative dementia such as Alzheimer's disease.
Dementia can also be caused by:
Alzheimer's dementia is caused by progressive brain cell death. Estimates range between 60% and 80% for the proportion of all cases of dementia being accounted for by Alzheimer's disease. In the US, about 5.3 million people are thought to have the disorder among the estimated 6.8 million individuals who have some form of dementia.