Agent 09 : Medicine. A factor, such as a microorganism, chemical substance, or form of radiation, whose presence, excessive presence, or (in deficiency diseases) relative absence is essential for the occurrence of a disease
Cornell University, Department Neurobiology and Behavior, Course: Clinical Neurobiology, Glossary of Epidemiological Terms, New York, USA 2012. https://courses.cit.cornell.edu/bionb4280/Epidemio logy%20Terms.pdf, (Dzstr Prep/ Epid - Amer) {1004}