Delusional disorder is characterized by a single fixed, unshakeable, FALSE belief that could be plausible, but is not in keeping with the person's social, cultural, and religious background. The term Paranoia is used to denote a thought process heavily influenced by anxiety or fear of persecution to the point of irrationality and delusion. 

A person with delusional disorder may be highly functional in daily life and may not exhibit odd or bizarre behavior other than the delusion. The family,  friends, and colleagues may actually believe that what the person is saying is true till a major incidence convinces them that it is not so e.g. the family of a person with delusional jealousy may actually believe that the spouse is unfaithful, till something happens to convince them that she is not, or the family of a person with a delusion of persecution may actually believe that he / she is being discriminated against at work, till they begin to suspect something wrongs after the person keeps quitting jobs within a few months, and they find out otherwise from colleagues.

The condition is difficult to treat because:

• The person is fully functional and comes up with plausible explanations for the belief that family and friends believe.
• Because of the above, the person refuses to seek treatment.
• Even is the person is taken for treatment, he / she usually discontinues the treatment within a short time.

Common Delusions (any one) that can comprise a Delusional Disorder:

• Delusion of Love (erotomania): The delusion that another person is in love with the individual. The individual tries to obsessively make contact with the desired person.
• Delusion of Jealousy:  The delusion that the individual's sexual partner is unfaithful when it is untrue. The individual may follow the partner, check text messages, emails, phone calls etc. in an attempt to find "evidence" of the infidelity.
• Delusion of persecution: The delusion that the individual is being malevolently treated in some way. The individual may believe that he/she is being harassed, discriminated and plotted against, talked badly about etc., when it is not true.
• Somatic delusion: The delusions that the person has some bodily imperfection  or general medical condition when it is not true.

Delusional disorder must be differentiated from Schizophrenia.

Treatment is with medication followed by psychotherapy AFTER the delusion begins to be shakeable and patient is open to discuss it.