Paranoia involves intense anxious or fearful feelings and thoughts, often related to persecution, threat, or conspiracy.
Paranoia can develop into delusions, when irrational thoughts and beliefs become so fixed that nothing (including contrary evidence) can convince a person that what they think or feel is not true.
When a person has paranoia or delusions, but no other symptoms (like hearing or seeing things that aren’t there), they might have what is called a delusional disorder. Because only thoughts are impacted, a person with delusions can usually work and function in everyday life, however, their lives may be limited and isolated.
Paranoia occurs in many mental disorders, but is most often present in psychotic disorders.