Paranoia refers to the perception or suspicion that others have hostile or aggressive motives in interacting with them (for example, "they are out to get me"), when in fact there is no reason for these suspicions. People experiencing paranoia believe that others are persecuting them and have delusional ideas about themselves as central figures in scenarios that in reality have little relevance to them. They may mistrust others and remain often in a state of suspicion. Minor feelings of paranoia are common, but severe paranoia can cause significant fear and anxiety and can have a pronounced effect on social functioning. Feelings of paranoia can be observed with many psychological disorders , including schizophrenia , as well as with a number of medical diseases that can affect brain function, ranging from Alzheimer's disease to multiple sclerosis . Intoxication from alcohol or drug abuse may also lead to feelings of paranoia.