Paranoia, or persecution mania, is the conviction that one or more people are trying to injure, kill or in some other way harm the person concerned. Everybody misinterprets other people’s behaviour on occasion, and feels himself unjustly treated or cheated. An idea of this kind can take on the nature of a delusion in certain mental illnesses such as schizophrenia, psychosis and dementia. Paranoia occurs principally when the sense of reality is reduced or lost. The delusion is a means by which the patient ‘explains’ fears, ideas and events which he does not understand. For someone who does not think himself ill a psychiatric investigation can seem like proof that he is being held prisoner. Someone who cannot or will not see that he is demented will not ascribe the fact that he keeps losing things to his own forgetfulness, but will think that someone is stealing from him. Any delusion is a part of a more general syndrome; thus treatment depends on the particular illness. In general it is necessary to enlist medical assistance. One problem of paranoia is that patients often see medication as attempted poisoning, and may thus refuse treatment.