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Progressive loss of memory is the dire result of Alzheimer's disease, a degeneration of the brain that generally strikes in old age. This, too, is an irreversible loss.
A head injury can also cause amnesia, but often the effect is temporary. For example, the victim of a car crash may lose memories of events just prior to the accident. Scientists call this retrograde amnesia.
Severe emotional stress is another cause of amnesia. Victims of crime and battle-scarred soldiers may protect themselves from the horrors they have experienced by blotting out the memory.. Such a mechanism may seem merciful, but can result in deeper emotional trouble, for, it seems, these memories are not lost. Unless the past is retrieved and faced, the stress may fester.