Delusional disorder
Delusional disorder is a psychiatric diagnosis denoting a psychotic mental illness that involves holding one or more non-bizarre delusions in the absence of any other significant psychopathology (signs or symptoms of mental illness). In particular, a person with delusional disorder has never met any other criteria for schizophrenia and does not have any marked hallucinations, although tactile (touch) or olfactory (smell) hallucinations may be present if they are related to the theme of the delusion.
A person with delusional disorder can be quite functional and does not tend to show any odd or bizarre behavior except as a direct result of the delusional belief. "Despite the encapsulation of the delusional system and the relative sparing of the personality, the patient's way of life is likely to become more and more overwhelmed by the dominating effect of the abnormal beliefs". (Munro, 1999)
Types
Grandiose Type: delusion of inflated worth, power, knowledge, identity, or special relationship to a deity or famous person (e.g.
see Jerusalem syndrome)
σ.σ. μερικοί το παθαίνουν μετά από απλή επίσκεψη σε ανώτερη σχολή (και χωρίς κατ’ ανάγκη να έχουν αποκομίσει γνώσεις πόσο μάλον ουσίας 
)
Indicators of a delusion
(Munro, 1999)
The patient expresses an idea or belief with unusual persistence or force.
That idea appears to exert an undue influence on his or her life, and the way of life is often altered to an inexplicable extent.
Despite his profound conviction, there is often a quality of secretiveness or suspicion when the patient is questioned about it.
The individual tends to be humorless and oversensitive, especially about the belief.
There is a quality of centrality: no matter how unlikely it is that these strange things are happening to him, the patient accepts them relatively unquestioningly.
An attempt to contradict the belief is likely to arouse an inappropriately strong emotional reaction, often with irritability and hostility.
The belief is, at the least, unlikely.
The patient is emotionally over-invested in the idea and it overwhelms other elements of his psyche (psychology).
The delusion, if acted out, often leads to behaviors which are abnormal and/or out of character, although perhaps understandable in the light of the delusional beliefs.
Individuals who know the patient will observe that his belief and behavior are uncharacteristic and alien.