Terrorism caused by chemical, biological or nuclear agents differs from terrorism using explosive forces. It is argued that ideal situations are created to attribute a wide variety of vague, systemic symptoms to environmental stimuli. A series of laboratory studies is discussed showing symptom learning in response to odorous chemical substances: the experience of a few symptom episodes in association with such substances is sufficient to subsequently feel symptoms upon perceiving the substances alone. This is more likely when the substances are foul smelling or endowed with negative meanings. Persons tending to negative moods and emotions appear more vulnerable to these processes. Implications for medically unexplained symptoms are discussed.