Jealous type: delusion that the individuals 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 they have been drugged, spied upon, harmed, harassed and so on and may seek "justice" by making reports, taking  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:
An individual expresses an idea or belief with unusual persistence or force, even when evidence suggests the contrary. That idea appears to have an undue influence on the persons life, and the way of life is often altered to an inexplicable extent. Despite their profound conviction, there is often a quality of secretiveness or suspicion when the person 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 the person, he or she accepts them relatively unquestioningly.
"Resist the negation reflex: Minimizing reactance in psychotherapy of delusions" (PDF). Psychosis. 8 (2): 1–10. doi:10.1080/17522439.2015.1095229. S2CID 146386637. McDermott, Sarah (22 February 2018), "The story of a weird world I was warned never to tell", BBC News. [A related case study.] Munro, A. (1999) Delusional Disorder: Paranoia and Related Illnesses. Cambridge: Cambridge University Press. ISBN 0-521-58180-X. Sims, A. (1995) Symptoms in the mind: An introduction to descriptive psychopathology. Edinburgh: Elsevier Science Ltd. ISBN 0-7020-2627-1. External links

 Media related to Delusional disorders at Wikimedia Commons