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Psychosomatic illnesses or symptoms refer to physical conditions which are psychologically induced. If a person's mind believes strongly enough that they are unwell, their body may create physical symptoms in response. Note that this does not imply that the person is faking the illness.
It is often difficult for doctors to determine which conditions are psychosomatic and which are genuine. Often psychosomatic illness is proposed as a hypothesis when there is no identifiable cause for a patients' condition.
Some theories about the human body's psychosomatic abilities assert, that in extreme cases, the body can even produce the appearance of wounds. This is proposed as one possible explanation for the phenomenon of stigmata.
Psychosomatic effects can be positive as well as negative, as demonstrated in the placebo effect. Placebos can be used successfully to treat psychosomatic conditions, as well as other minor or temporary illnesses.
According to Wikipedia, a psychosomatic ailment is more correctly called a somatoform disorder unless it's related to high blood pressure, stress, etc..
Multiple chemical sensitivity
Widespread chemophobia, coupled with the fact that some volatile compounds smell nauseatingly bad, has led a number of folks to believe that they have severe, allergy-like physical reactions to a wide variety of chemicals. When tested, many of these patients have no symptoms at all if their nose is plugged up and they're not aware that they're being exposed to one of the chemicals they're "sensitive" to.
Pyschosomatic illness is one topic of discussion for French philosopher Jean Baudrillard in his work Simulacra and Simulation. The book quotes Littre speaking on psychomatic illness and simulation: "Someone who feigns an illness can simply go to bed and make believe he is ill. Some who simulates an illness produces in himself some of the symptoms."  To Baudrillard, such illness is a form of simulation which denies treatment by medicine.
- Das-Munshi et al., Multiple chemical sensitivities: A systematic review of provocation studies, Journal of Allergy and Clinical Immunology, vol. 118 no. 6 (Dec 2006), pp. 1257-1264