Infectious diseases caused by bacteria and viruses exist in many forms and significantly affect human health. The sources of infectious diseases are vast, but in most cases arise from infectious microorganism such as bacteria or viruses that are able to establish growth or replication in humans, harming specific systems of the human body. This book introduces the reader to the basic differences between bacteria and viruses, particularly focusing on structures that contribute to the infectious properties of the microorganism. Chapters describe the cause, mode of transmission, symptoms, and treatments of five important diseases, taking into consideration the molecular interactions between host cells and infectious agents. Specifically, examples of viral infection (Influenza caused by the Influenza virus and hemorrhagic fever caused by the Ebola virus) and specific examples of bacterial infections (salmonellosis caused by Salmonella, gastrointestinal disease caused by Shiga-like toxin E. coli, and tuberculosis caused by Mycobacterium tuberculosis) are discussed in each chapter. The book ends with some future work related to treatment of these critical infectious diseases, noting the importance of drug resistance of infectious agents in treatment regimens.