1) Alzheimer's disease is a degenerative brain disorder and the most common form of dementia that usually starts in late middle age or old age, resulting in progressive memory loss, impaired thinking, and disorientation. 2) It is a chronic and irreversible disease that affects brain cells and causes impairment of intellectual functioning, destroying the ability to reason, remember, imagine and learn over time. 3) The causes of Alzheimer's disease are unknown but several factors have been implicated including environmental factors like smoking, infections, and toxins as well as genetic and immunological factors.