Which condition is a cause of prosopagnosia?

I have a mild version of this condition and it can be quite annoying and embarrassing when I meet people I have met before but I'm not able to recall their faces and I come across as rude. I'm a teacher and I sometimes don't even recognise my students when I see them out of the familiar context of the classroom. It also makes it pretty difficult to watch films as I have problems recognising characters across different scenes. Sigh.

Ania, I suffer with the same mild form..people greet me warmly whilst I am out shopping and I don't recognize them and don't know how warmly I should reply to them. Also it seems rude to ask them 'who they are?'