The more common name for this is “face blindness” and it can be either inherited or acquired following some type of brain damage (for example a stroke or developing brain lesions.)
But to be more precise, it’s a neurological deficit rather than a mental illness.
Jane Goodall has it as does Oliver Sachs, a noted neurologist (as well as a few members of his family). Would they be considered “mentally ill”? I rather doubt it.
There really is go standard effective treatment and most people who have this develop various adaptive techniques.
In this way it’s not unlike dyslexia (also a neurological problem of perception but NOT a mental illness) and dyslexics find different methods to adapt to their deficit.
I don’t know that this is necessarily so frightening since many people with the congenital (inherited) form spend many years not knowing they have this.
Even tho he knew what this condition was (due to his profession as a Neurologist) Oliver Sachs didn’t recognize this in himself until it was pointed out to him by others.
So, it’s obvious it wasn’t scary to him since he didn’t even know he had it :)
I can think of a lot of scarier conditions to deal with but I guess it depends upon each individual and how they react.
I’ve heard several interview of people with face blindness and none of them sounded scared to me. To the contrary, they were usually quite proud of the various adaptive methods which they developed to deal with this deficit.