There is a specific mental condition that blocks the ability to recognize illness in oneself. It works whether the illness is schizophrenia, paralysis, or even complete blindness. It's called anosognosia.
Apparently one of the major problems dealing with people just after they've had a stroke is convincing them that they've had a stroke – or rather, that the stroke has had any effect on them. In one study, nearly 60% of people who had just had a stroke that had left their right side partially or totally paralyzed did not admit to any kind of paralysis. People simply said they were disinclined to move their arms or legs. They came up with reasons why they refused doctors' requests to move or walk.
This was not a refusal to acknowledge problems, but an actual inability to recognize them. The patients recognized that they weren't moving their arm, and their brain came up with a reason why their arm wasn't moving. Either they were tired, or didn't feel like moving it, or were affronted by the request.
They had anosognosia. It can happen when any part of the brain is affected, but for some reason it seems especially prevalent in patients who have strokes on the right side of the brain. In each case, it's the lack of ability to see any problem – no matter how clear the illness.
To be fair to those who do have anosognosia, it's easier to believe that one is not interested in obeying a doctor than believing that one has suffered a serious injury or stroke. When I stay in bed in the morning, well past when my alarm clock goes off, the most likely explanation for my behavior is I'm lazy, not that I'm unable to rise. Patients with anosognosia are engaging in an extension of what we do every day, explaining our behavior. Their brain supplies them with the most common and likely answer to why they don't comply with a physician's requests.
The anosognosia kicks in when, despite repeated failure to act, they still don't realize that they're injured. One woman went blind, and spent quite some time saying that her vision was merely blurry, or that she hadn't expected objects to be where they were, or that she was just clumsy. She had to heal before she could accept that she was sick.
Anosognosia has a physical cause – generally it's a lesion in the brain, and it can be treated temporarily. Amazingly, people will have moments of clarity due to nothing more than a squirt of cold water in the ear. The irrigation of the ear, which is sometimes done to test for brain death, will stimulate nerves in the brain and cause them to fire. The patient will regain some brain function, temporarily, and show a reaction. In patients with brain damage, the eyes will turn toward the ear. In patients with anosognosia, the person will suddenly realize their lack of abilities.
The effect doesn't last. There is no long term way to eliminate anosognosia. It takes time, and experience, for the brain to find ways to acknowledge the reality of the situation.