This is a more serious form of depression which is frequently seen by psychiatrists though it is rather rare, compared to the wider context of general medical practice. It shares many of the features of psychiatric disorders traditionally considered as serious.
Patients may develop beliefs about themselves and the world, which are false at times bizarrely and obviously so. Typically sufferers of severe depression may become convinced that they are guilty of shameful deeds or crimes which they could not possibly be responsible for, such as a war or a natural disaster.
They may believe that they are suffering from a terminal illness, and may interpret other symptoms of depression, such as weight loss or constipation, as proof of this. They may feel that others view them as worthless and that they deserve their torment as a just punishment.
Consistent with this belief, sufferers may hallucinate voices which might scorn, insult or threaten them, or even urge them to commit suicide, or they may see terrifying hallucinated apparitions of the devil for example.
Naturally such people may quite easily not recognize that they are ill, and because they are at risk of serious self-neglect through failing to eat or drink, or failing to care for their hygiene, or in some cases be greatly at risk of a suicidal act, they may require hospital admission. Sometimes for their own protection this is not on a compulsory basis. In these cases depression is a seriously debilitating and potentially life-threatening illness and recovery from it is often delayed and incomplete.