Dementia is a clinical syndrome that includes loss or decline in memory and other cognitive abilities. Dementia is caused by various diseases and conditions that result in damaged brain cells. Brain cells can be destroyed by brain diseases, such as Alzheimer’s disease, or strokes (called vascular or multi-infarct dementia), which decrease blood flow to the brain. Alzheimer’s disease is the most common cause of dementia. Subscribe to The Alzheimer’s Reading Room–via Email Dementia can be caused by any of the following: AIDS, high fever, dehydration, hydrocephalus, systemic lupus erythematosus, Lyme disease, long-term drug or alcohol abuse, vitamin deficiencies, poor nutrition, hypothyroidism or hypercalcemia, multiple sclerosis, brain tumor.