Interactions between competitors, predators and their prey have traditionally been seen as the foundation of community structure. Parasites - long ignored in community ecology and the care necessary bio-security are now recognized to play an important role in influencing species interactions and, consequently affecting the functioning of the ecosystem. Parasitism can interact with other ecological drivers, resulting in both adverse and beneficial effects on biodiversity and ecosystem health. Species interactions involving parasites are also keys to understanding many biological invasions and emerging infectious diseases. This book bridges the gap between ecology and epidemiology community to create a comprehensive examination of how parasites and pathogens affect all aspects of ecological communities, enabling a new generation of environmentalists to include parasites as a key element in their studies may be parasites human or animal, terrestrial or aquatic environments.