Paranoia is a personality disorder characterized by an overwhelming feeling of distrust and suspicion of other people and their motives. Those suffering from paranoia may feel that people are trying to threaten or malign them.
Paranoia and paranoid disorders (such as paranoid personality disorder and paranoid schizophrenia) are believed to be caused by a combination of biological and environmental factors.
Paranoia can manifest in many ways, including some very specific conditions. For example, someone suffering from erotomania may become convinced that a stranger -- often times someone of public importance -- is in love with him or her. And, people with hypochondriacal paranoia believe that they're suffering from a serious medical disease yet their doctors are persecuting them by refusing to acknowledge the illness and treat it.