Bacteria and plants produce powerful toxins that can cause a variety of diseases, some of which are lethal for many animal species. The mechanisms of action are common to many of these toxins and represent general pathways for the interaction of a number of biomolecules with target cells such as binding to specific surface receptors, internalization and translocation across the cell membrane and interaction with intra-cellular components. Because of their potency in killing eukaryotic cells, they have been used in the development of novel agents for targeted immunotherapy and have been termed...