This book contains material relevant for students and researchers interested in the basic science of infectious diseases. The material in this book is organised into three major sections. The first describes the structures of various pathogenic microbes. The emphasis in this section is on the surface structures as they mediate the critical step of adsorption of pathogens to their target host cells. There is also a subsection on protein secretion in bacteria. The second section reviews the basics of innate immunity as these are the first line of immune defence and are important to understand the immunological response and its consequences on the health status and physiological balance in the host. The third section provides a description of a number of infectious diseases, particularly those of the respiratory system and prion-related infections.