Disease prevention is the prime motivation behind the development of vaccines, and as one disease is conquered, researchers strive to add to their knowledge and utilize new technologies to control and eradicate diseases for which, heretofore, vaccines have been elusive. This effort drives the research toward a fuller understanding of the human immune system and the nature of pathogenic microorganisms, as well as the development of new vaccines. This title examines the science behind vaccine research in the following chapters: What Are Vaccines? Vaccines and Viruses; Vaccines and Bacteria; Safe, Effective, and Approved; Innovative Vaccines for the Future.