Delusional disorder is a mental illness in which the patient presents with delusion and a classical case study of a Political Party in Malaysia suffering acutely for this disease

Delusional disorder is a mental illness in which the patient presents with delusions, but with no accompanying prominent hallucinations, thought disorder, mood disorder, or significant flattening of affect. Delusions are a specific symptom of psychosis. Delusions can be "bizarre" or "non-bizarre" in content; non-bizarre delusions are fixed false beliefs that involve situations that could potentially occur in real life, such as being followed or poisoned .Apart from their delusions, people with delusional disorder may continue to socialize and function in a normal manner and their behavior does not necessarily generally seem odd. However, the preoccupation with delusional ideas can be disruptive to their overall lives. For the diagnosis to be made , auditory and visual hallucinations cannot be prominent, though olfactory or tactile hallucinations related to the content of the delusion may be present. 

To be diagnosed with a delusional disorder, the delusion(s) cannot be due to the effects of a drug, medication, or general medical condition, and delusional disorder cannot be diagnosed in an individual previously properly diagnosed with schizophrenia. A person with delusional disorder may be high functioning in daily life. Recent and comprehensive metaanalyses of scientific studies point to an association between a deterioration in aspects of IQ in psychotic patients, in particular perceptual reasoning.

According to German psychiatrist Emil Kraepelin, patients with delusional disorder remain coherent, sensible and reasonable.



The Diagnostic and Statistical Manual of Mental Disorders (DSM) defines six subtypes of the disorder characterized as erotomanic (believes that someone is in love with them), grandiose (believes that they are the greatest, strongest, fastest, richest, and/or most intelligent person ever), jealous (believes that the love partner is cheating on them), persecutory (delusions that the person or someone to whom the person is close is being malevolently treated in some way), somatic (believes that they have a disease or medical condition), and mixed, i.e., having features of more than one subtype. Delusions also occur as symptoms of many other mental disorders, especially the other psychotic disorders.

The DSM-IV, and psychologists, generally agree that personal beliefs should be evaluated with great respect to cultural and religious differences, since some cultures have widely accepted beliefs that may be considered delusional in other cultures.

Classification

Diagnosis of a specific type of delusional disorder can sometimes be made based on the content of the delusions. The Diagnostic and Statistical Manual of Mental Disorders (DSM) enumerates seven types:

Erotomanic type (erotomania): delusion that another person, often a prominent figure, is in love with the individual. The individual may breach the law as he/she tries to obsessively make contact with the desired person.

Grandiose type: delusion of inflated worth, power, knowledge, identity or believes himself/herself to be a famous person, claiming the actual person is an impostor or an impersonator.

Jealous type: delusion that the individual's sexual partner is unfaithful when it is untrue. The patient may follow the partner, check text messages, emails, phone calls etc. in an attempt to find "evidence" of the infidelity.

Persecutory type: 

This delusion is a common subtype. It includes the belief that the person (or someone to whom the person is close) is being malevolently treated in some way. The patient may believe that he/she has been drugged, spied upon, harassed and so on and may seek "justice" by making police reports, taking court action or even acting violently.
Somatic type: delusions that the person has some physical defect or general medical condition
Mixed type: delusions with characteristics of more than one of the above types but with no one theme predominating.

Unspecified type: delusions that cannot be clearly determined or characterized in any of the categories in the specific types,

Signs and symptoms, 

The following can indicate a delusion

The patient expresses an idea or belief with unusual persistence or force.

That idea appears to have an undue influence on the patient's life, and the way of life is often altered to an inexplicable extent.

Despite his/her 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, and out of keeping with the patient's social, cultural and religious background.

The patient is emotionally over-invested in the idea and it overwhelms other elements of their psyche.

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 observe that the belief and behavior are uncharacteristic and alien.

Additional features of delusional disorder include the following
It is a primary disorder.

It is a stable disorder characterized by the presence of delusions to which the patient clings with extraordinary tenacity.

The illness is chronic and frequently lifelong.

The delusions are logically constructed and internally consistent.

The delusions do not interfere with general logical reasoning (although within the delusional system the logic is perverted) and there is usually no general disturbance of behavior. If disturbed behavior does occur, it is directly related to the delusional beliefs.

The individual experiences a heightened sense of self-reference. Events which, to others, are nonsignificant are of enormous significance to him or her, and the atmosphere surrounding the delusions is highly charged.

Read below a classic example of Malaysian Political party suffering from a case of mental Delusional disorder

PAS’ target of four states and 40 parliamentary seats not ‘empty talk’

PAS party strategist Dr Zuhdi Marzuki says the party's research backs up its claims that it will win up to five states and 40 parliamentary seats in the next elections. — The Malaysian Insight pic, May 1, 2017.

PAS is confident of being able to win four to five states and 40 parliamentary seats, based on its extensive internal studies, said party strategist Dr Zuhdi Marzuki.

“When the president says we target 40 parliamentary seats and we can rule four to five states, it’s not empty talk, but based on our own studies,” Zuhdi said at the party’s 63rd assembly in Alor Star, Kedah.

Zuhdi, however, did not reveal the results of these studies which included about 100 simulations and analyses from several of its think-tanks.

These research centres are based in Kelantan, Terengganu, Perak and Sabah.

“We looked at both scenarios, straight fights and three-cornered fights. At the same time, we do not disregard the studies of other think-tanks.” — May 1, 2017.