The development of vaccines is arguably the most significant achievement in medicine to date. The practice of innoculation with the fluid from a sore to protect from a disease actually dates back to ancient China; however, with the introduction of Jenner's smallpox vaccine, and greater understanding of the immune system, vaccines have become specific and systematic. Traditional vaccines have used killed pathogens (hepatitis A and the Salk polio vaccines), immunogenic subunits of a given pathogen (hepatitis B subunit vaccine), or live attenuated pathogens (measles, mumps, rubella, Sabin polio vaccines) to generate protective immunity. Currently, a new generation of vaccines that use the genetic material of a pathogen to elicit protective immunity are being developed. Although the most widespread and successful use of vaccines today remains in the arena of infectious diseases, manipulations of immune responses to protect against cancers, neurologic diseases, and autoimmunity are being explored rigorously.