Memory loss: what makes people forget who they are?

When amnesia strikes, people can forget everything about their life, including their name. But what causes memory loss? And what happens to people who lose themselves for an hour, a few months - or even for ever?

She was missing but police knew where she was. She could not remember her name, her family or her childhood. She knew that she was dying, but only that. Interpol released a missing persons report: 1.7m, 91kg, brown eyes, chip on front tooth, right-handed, Caucasian, appears to be in her 50s, piercing on each ear, shoe size 39. Languages: English, French.

She called herself "Sam" and spoke to the media this month, explaining that she had been found semi-conscious by police outside a church in Carlsbad, California, five months ago. She had stage three ovarian cancer, she said. A Facebook campaign earned 200,000 shares and ignited worldwide media interest. Then Sam's scattered recollections started to emerge: "" swimming in a salt water pool in Perth, then icebergs in New South Wales and in Cairns in Queensland and Byron Bay".