Psychosis is a severe mental disorder characterized by impaired thinking capability. People suffering from psychosis have false thoughts (delusions) and see or hear things that are not there (hallucinations). The combination of hallucinations and delusional thinking can often severely hamper perception, thinking, emotion and behaviour. Psychosis can be triggered by traumatic experiences, stress or physical conditions, such as Parkinson's disease, schizophrenia, bipolar disorder, a brain tumour, or as a result of drug misuse or alcohol misuse.