Prosopagnosia is a rare condition, usually brought on by some kind of damage to the brain that causes people to lose the ability to recognize faces.
This is a type of visual agnosia, which prevents people from recognizing objects or shapes. People with these conditions are functional in every other way, with normal intelligence and mental aptitude, but must adjust their way of perceiving through the world by relying on other senses, much like blind people.
With a little extra planning and care, these people can go on to live relatively normal lives.