Vaccines have proven among the most cost-effective strategies for preventing infectious diseases — following only the provision of safe drinking water and sanitation. During the 20th century, vaccines for bacterial toxins and many common acute viral infections were developed and made widely available. Vaccines have changed the face of viral disease as much as antibiotics have affected the course of a bacterial disease. New strategies for second-generation vaccines based on cellular immunity. Vaccines represent one of the greatest triumphs of modern medicine. The development of the first vaccine by Edward Jenner in 1796 to prevent infection by the smallpox virus was a watershed moment in the war against microbes. Over the next two centuries, human morbidity and mortality resulting from polio, measles, mumps, rubella, pertussis, and diphtheria have dramatically declined by over 95% due to the development of prophylactic vaccines. The human body has developed an elaborate defense system against microbial pathogens and foreign antigens. However, particular microbes have evolved sophisticated mechanisms to evade immune surveillance, allowing persistence within the human host. In an effort to combat such infections, intensive research has focused on the development of effective prophylactic and therapeutic countermeasures to suppress or clear persistent viral infections..