This course is the first of a two-course sequence covering both the etiologic agents of infectious diseases and the host immune responses to those agents. This course begins by focusing on fundamental principles of immunology, antigenic characteristics of microorganisms, the cells and mediators involved in host defense mechanisms against pathogens and tumor antigens, lymphatic recirculation, and lymphatic flow. Next, the course delves into the basic classification, structure, metabolism and genetics of bacteria, viruses, protozoa, helminths, and fungi. From that point on, lectures and small group activities use the organ systems approach to examine the etiologic agents of infectious disease, the immune system responses, and possible immunopathology. Clinical correlations and case presentations are featured for each organ system.