Epidemiology of dementia: Concepts and overview

Dementia is a relatively rare condition of midlife and a relatively common condition of late life, with both prevalence and incidence increasing with age. Alzheimer's disease is the most common cause of dementia; diagnostic criteria are still evolving for other subtypes. A number of risk and protective factors have been suggested by different types of studies reported in the literature. We have reviewed a selection of the literature on prevalence and incidence of dementia, and on risk and protective factors for dementia, emphasizing the relevant methodologic considerations. Results of well-designed risk and protective factor studies have causal implications, and can lead to trials of both prevention and treatment of the dementias.