Pathology is the study (logos) of suffering (pathos). It is a discipline that bridges clinical practice and basic science, and it involves the investigation of the causes (etiology) of disease as well as the underlying mechanisms (pathogenesis) that result in the presenting signs and symptoms of the patient. Pathologists use a variety of molecular, microbiologic, and immunologic techniques to understand the biochemical, structural, and functional changes that occur in cells, tissues, and organs. To render diagnoses and guide therapy, pathologists identify changes in the gross or microscopic appearance (morphology) of cells and tissues, and biochemical alterations in body fluids (such as blood and urine). Traditionally, the discipline is divided into general pathology and systemic pathology; the former focuses on the fundamental cellular and tissue responses to pathologic stimuli, while the latter examines the particular responses of specialized organs. In this book we first cover the broad principles of general pathology and then progress to specific disease processes in individual organs.