Alzheimer's disease is a type of dementia. Dementia presents with a loss of thinking, remembering, and reasoning skills. It generally creates problems with a person's daily life and activities. Alzheimer's disease is the most common cause of dementia among older people. Other types of dementia include frontotemporal disorders and Lewy body dementia.

Memory problems are usually one of the first signs of Alzheimer's disease. This is not the case for everyone. Each individual may have different initial symptoms. A decline in other aspects of thinking, such as finding the right words, vision/spatial issues, and impaired reasoning or judgment, may also signal the very early stages of Alzheimer's disease. Mild cognitive impairment, or MCI, is a condition that may be an early sign of Alzheimer's disease—but not everyone with MCI will develop Alzheimer's. In addition to memory problems, movement difficulties and problems with the sense of smell have been linked to MCI.

Alzheimer's disease progresses in several stages: preclinical, early (also called mild), middle (moderate), and late (severe). During the preclinical stage of Alzheimer's disease, people seem to be symptom-free, but toxic changes are taking place in the brain. A person in the early stage of Alzheimer's may exhibit the signs listed above. As Alzheimer's disease progresses to the middle stage, memory loss and confusion grow worse, and people may have problems recognizing family and friends. As Alzheimer's disease becomes more severe, people lose the ability to communicate. They may sleep more, lose weight, and have trouble swallowing. Eventually, they will need total care.