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Dementia is a decline in a person's ability to think and learn. It is an irreversible mental condition. Occurring mainly in older people, dementia is characterized by memory loss, the inability to concentrate and make judgments, and the general loss of other intellectual abilities.

The two most common forms of dementia are senile dementia and Alzheimer's disease. Senile dementia, or senility, is the loss of mental capacities as a result of old age. It is considered a normal part of the aging process, and generally occurs very late in life. Alzheimer's disease, on the other hand, is not a normal result of aging and can begin in late middle age.

The deterioration of brain tissue occurs much more quickly in those people suffering from Alzheimer's disease than in those suffering from senility. Alzheimer's disease is marked first by forgetfulness, followed by memory loss and disorientation, then by severe memory loss,...