Hallucinations can affect all of a person's senses. People with the mental illness of schizophrenia may hear sounds or voices that are not really there. People with certain mental illnesses may also have hallucinations in which they see a person or thing which is not really there. People are less likely to have hallucinations in which they smell or touch something that is not really there. Certain forms of epilepsy can cause hallucinations. Finally, hallucinations can be the result of certain substances taken or of certain physical constellations. Most of the time, these substances are taken to get the hallucinations. Sometimes, (severe) lack of sleep, or high fever can lead to hallucinations brought on by such viruses as the common cold.