The immune system is the only organ system in the body besides the central nervous systems endowed with memory. Both types of memories are specific and long-lasting, sometimes life long. This memory capacity of the immune system provides the basis for the most cost-efficient of all medical interventions, successful vaccinations against many common infectious diseases. Such a success requires the isolation of the infectious agent or toxic substance, methods to grow and/or purify the relevant antigen and change it into something innocuous whilst maintaining its immunogenicity. Whereas the early vaccines could only use the enhanced resistance against infectious disease as a measure of vaccine efficacy, most modern vaccines rely upon standardized laboratory tests accepted to parallel the in vivo protective capacity to confirm the quality and potency of the respective vaccine. We are presently experiencing an explosion in the development of new and/or improved vaccines. This is largely due to a parallel rapid expansion in our knowledge of the immune system and of the detailed molecular structure and function of microorganisms. Using this knowledge it is now possible to compose vaccines of new types where only certain molecules (or parts of molecules) derived from a pathogen are included, excluding other potentially harmful ones. Whereas earlier attenuated live vaccines were created by em­ pirical means such vaccines can now be created by molecularly defined inter­ ventions in the genome of the microorganism.