Vaccination is a clinical application of immunization designed to artificially help the body to defend itself. A vaccine against infection is a modified form of a natural immunogen, which may be either the whole pathogen, one of its components, or a toxin. A vaccine does not cause disease when administered but induces the healthy host (the vaccinee) to mount a primary response against epitopes of the modified immunogen and to generate large numbers of memory B and T cells. In an unvaccinated individual, (left panel), naïve B and T cells capable of combatting an infecting pathogen are present in relatively low numbers when the pathogen is first encountered. A primary immune response is all that can be mounted so that, in many cases, the individual becomes sick until antibodies and/or effector T cells can act to clear the attacker. In a vaccinated individual (prophylactic vaccination because it is intended to prevent disease.