Dementia refers to the loss of cognitive functions (thinking, reasoning, the ability to remember) that are severe enough to interfere with a person’s daily functioning. This group of symptoms is not a disease per se, but may accompany certain diseases or conditions. Dementia is irreversible when caused by disease or certain injuries. It may be partially or fully reversible when caused by drugs, alcohol, depression or imbalances in certain substances, such as hormones or vitamins.  Alzheimer’s disease is the most common form of dementia. It is a progressive, degenerative disease of the brain, characterized by loss of function and death of nerve cells in several brain areas. This initially leads to loss of recent memories and new learning, with old memories eventually fading as well.  Other common types of dementia include vascular dementia, dementia from Parkinson’s disease and dementia with Lewy bodies.