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Halogens have a bad reputation. These elements are found in dioxins, polychlorinated biphenyls (PCBs) and the pesticide DDT. With such infamous company, many people—even many scientists—believe all halogenated chemicals are uniquely man-made poisons. This assumption is false, according to Dartmouth chemist Gordon W. Gribble. In fact, protists, plants and animals—even humans—make thousands of distinct halogen compounds for their own defensive purposes. Some species even use organohalogens, which contain carbon along with chlorine, bromine, iodine or fluorine, to mount chemical offensives against encroaching competitors. Gribble highlights many remarkable examples of these natural halocarbons, and describes several that, like class-members morphine, penicillin and quinine, show promise as powerful medicines.