Alzheimer's disease is the most common form of dementia (decline in brain function) in people over 65. Delirium is severe confusion (usually curable) caused by physical or mental illness, including infections, poisoning and drugs. Delirium tremens results from alcohol withdrawal. Short-term memory loss can be caused by depression or medication. Vascular dementia and retrograde amnesia can be caused by stroke or other brain injury. Early-onset dementia affects people under age 65.