Vaccination is one of the most efficient and cost effective methods of promoting human health and has been in clinical use for at least 200 years. Nevertheless, infectious diseases continue to constitute a constant threat to the well being of humanity. Common pathogens, once believed to be under control, acquire increased virulence and resistance to drugs, while exotic microorganisms emerged from hidden reservoirs to cause yet incurable diseases in humans. These changes, together with epidemic outbreaks related to political and socio-economic instabilities, increase the needs for the...