It is a lack of self awareness. It is considered a neurological disorder that is different from someone who is simply in denial that they have a mental condition. In the case of someone with Anosognosia, a person with dementia or Alzheimer’s disease may not realize or be aware of the fact that they even have the conditions. No research has proven what causes it yet.
It’s also possible that a person may be aware of one disease but completely unaware of a different disease or ailment. Many people with dementia or Alzheimer’s develop Anosognosia and even when pointing out their disease to them, will deny having the disease at all. Not because they are ‘in denial’, but because they literally believe there is nothing wrong with them at all.