ABSTRACT
Pathogenic organisms are normal components of all ecosystems. Simply defined, pathogenic organisms are life forms that cause disease in other life forms. Most are parasites that live at the expense of their host. Some are free-living but cause disease indirectly, such as by producing toxins to which other life forms become exposed. Ecotoxicology is generally considered to be the study of chemical pollutants in ecosystems. However, pathogenic organisms and the diseases they cause are relevant in this context in several different ways. Pathogens can be regarded as pollutants when they are released by humans into ecosystems for the first time or when they are concentrated in certain areas by human activity. Human activities also can alter ecosystems in ways that change the relationships among pathogens and their hosts and, thus, the occurrence of diseases. The transportation of plants, animals, and human beings around the world has created many new associations of potential pathogens, hosts, vectors, and environments out of which a substantial number of new or newly important diseases have recently emerged. The technology of genetic engineering has made possible the creation of whole new strains of pathogens that are indeed man-made.
