Psychosis is a term used to describe a severe mental illness. Psychoses are characterized by a variety of symptoms that most people consider abnormal. These include hallucinations, such as hearing voices when no one is around, and delusions, such as the notion that one is being persecuted or conspired against. 

Some psychoses have obvious physical causes. For example, the brain may be diseased as the result of an infection such as general paresis, which is caused by syphilis, a venereal disease. Or, a physical illness of another part of the body may affect the brain, as in delirium due to pneumonia. These are called organic psychoses. In toxic psychoses, a harmful or poisonous substance (toxin) affects the brain. An example of this is a psychosis caused by lead poisoning. Other psychoses have no obvious physical cause. The most common psychosis is schizophrenia.