Loss of ability to memorize information and/or to recall information stored in memory. Possible causes of amnesia are head injury; degenerative disorders such as Alzheimer’s disease and other forms of dementia; infections such as encephalitis; thiamine deficiency in alcoholics, leading to Wernicke–Korsakoff syndrome; brain tumours; strokes; and subarachnoid haemorrhage. Amnesia can also occur in some forms of psychiatric illness. In retrograde amnesia, the loss of memory extends back for some time before the onset of the disorder. In anterograde amnesia, there is an inability to store new information in the period following the onset of illness.