The third fully revised edition of this book introduces the biology of parasitic single-celled organisms, worms, and arthropods. The authors provide an overview of the various groups of parasites based on current taxonomy. Important pathogens, such as those causing malaria, sleeping sickness, and toxoplasmosis, are discussed in detail, with explanations of the molecular foundations of these pathogens. Additionally, parasites are presented as disease vectors, and significant infectious diseases affecting animals are addressed. Therefore, this book is aimed at both biologists and veterinarians as well as medical professionals. Each chapter discusses adaptations to a parasitic lifestyle. Typical life cycles, immune responses, and the resulting disease manifestations are illustrated using common representatives.