A hallucination describes any auditory or visual sensation that is not rooted in physical reality, such as hearing voices or seeing bugs on the skin. Auditory hallucinations are very common - over 80% of people have experienced at least one. Visual halluciations are somewhat more rare. Neither is a sign of serious illness if they are an occasional occurrence in an otherwise healthy person - they can commonly occur when a person is overtired or is suffering from sensory deprivation, such as driving in a remote area at night.
However, frequent hallucinations, or those that appear impossible for the patient to ignore, are usually the sign of a serious illness. Persons with schizophrenia often have persistent and disturbing auditory hallucinations. Persons using certain drugs, such as LSD or mescaline, also commonly have hallucinations. Hallucinations are also common in people suffering from severe fever. However, absent these conditions, hallucinations are usually the result of a physical illness of the brain.
In Cane & Able, a boy's condition makes him perceive normal situations as alien encounters.
In Who's Your Daddy?, a girl's fungal infection causes her to hallucinate that rooms are filling up with water.
In Autopsy, a young cancer patient starts suffering hallucinations due to a blood clot in her brain.
In Acceptance, a multiple murderer hallucinates that his victims are in the exercise yard with him.