Prosopagnosia is a disorder of the brain where a person becomes unable to recognize faces, the way any other person could. A person can have a severe form where they are completely unable to recognize a face or they can have it where they only have slight difficulty recognizing a familiar face. This, in my opinion, is one of the scariest disorders. I can't imagine not being able to recognize my friends or family members or even my OWN face. There is no "cure" for this disorder but people with it usually try and remember clues about people's faces this way it helps them to recognize them easier. Not being able to recognize people around you that you normally would recognize can leave you feeling scared and if you don't do something to help this disorder you will have to live not being able to recognize people forever.
Here is a video of ways to help cope with face blindness: