Vaccines have historically been the most effective means to provide active acquired immunity and fight against infectious diseases. Nowadays, vaccines have been developed either as a prophylactic treatment to provide active acquired immunity or as a therapeutic. A vaccine typically contains an agent that stimulates the body's immune system to recognize the agent as a threat, destroy it, and keep memory of it. These agents are commonly based onpurified protein antigens from natural origin or produced by recombinant DNA, polysaccharides, semi-synthetic poly- or oligosaccharide-protein conjugates and novel nucleic acid constructs.