When first confronted with the emergent threat of bioterrorism I used a mental paradigm based on the principles of nidality. Instead of considering the biological niche of the organism in nature, however, I chose to define the agent's nidality from an epidemiological perspective. Every organism that causes disease in human populations has an expected epidemiologic presentation. The epidemiologic pattern provides clues as to whether the clinical presentation is "expected" or "un-expected" in that physician's range of practice experience. A corollary to the expected/un-expected clinical paradigm is the potential route of exposure to the organism in a particular geographic and/or occupational setting (6) . Routes of exposure can be categorized as "natural" or "un-natural" and may in fact influence the constellation of signs and symptoms observed by the clinician. I now employ these concepts in teaching and instructional activities to prepare health care providers in biodefense preparation and response.