Alzheimer's isease (AD), also called Alzheimer disease or simply Alzheimer's, is the the most common cause of dementia. A Delphi study formed an expert consensus estimate that 24 million people worldwide had dementia in 2006, and projected a doubling every twenty years. Alzheimer's is a degenerative and terminal disease for which there is no known cure. In its most common form, it afflicts individuals over 65 years old, although a less prevalent early-onset form also exists. The disease can develop many years before it is eventually diagnosed. In its early stages, memory loss, shown as a difficulty to remember recently learned facts, is the most common symptom, although it is often thought to be caused by aging or stress by the sufferer at first. Later symptoms include confusion, anger, mood swings, language breakdown, long-term memory loss, and the general withdrawal of the sufferer as his or her senses decline. The sufferer gradually loses minor and major bodily functions leading to death. Each individual experiences the symptoms of AD in unique ways. The duration of the disease is between 5 and 20 years.