Prosopagnosia is an unknown disorder of  face perception. It is also called face blindness, which means, the person who suffers from it is unable to recognize faces.
Many people don't know about prosopagnosia. Some might not even know that they have this disorder. 2 percent of the world's population, however, is suffering from it. Face blind people are not even able to recognize their own parents. If they see a person, they won't remember how they look like when they're gone and then won't recognize them even though they see them daily. 

Those suffering from prosopagnosia often feel ashamed when not recognizing friends or business partners etc. Many people keep their disorder a secret. It has only been a few years ago since the disorder has been discovered and determined.