Face blindness: Seeing but not seeing
There are two main forms of prosopagnosia; developmental - where people fail to develop face processing abilities which is thought to affect around 2% of the population, and acquired, which develops after some form of brain injury and is much rarer.
Dr Ashok Jansari, a cognitive neuropsychology expert from the University of East London said: "Acquired prosopagnosia is extremely rare because the type of brain damage is very specific,.
"It can be caused by damage to the back of the brain on the right-hand side in an area known generally as the right occipito-temporal region."
David said: "I don't know what's worse - not being able to ever recognise people or at 56 years old - as it happened to me - suddenly not being able to recognise anybody".
He added that the worst part is the social embarrassment.
"We were on holiday in Cuba and I'd been snorkelling in the sea. I was talking with this guy from Denmark, when this woman swam up and said 'buenos dias' and I went 'hello, pleased to meet you', thinking it was his wife, but it was actually my wife and I hadn't recognised her."
David can see people perfectly well - but 10 or 15 minutes later he cannot recognise them.
He now tells clients: "If I ignore you, I'm not being rude, it is just that I can't recognise you."