The exponential growth in vaccine research over the last decade, in which many infectious diseases now appear to be amenable to prevention through immunization, is built upon three factors: first, a richer understanding of the immune response (in particular, cellular immunity), second, a greater finesse in understanding the molecular biology of pathogenicity, and third, an expanding use of genetic engineering techniques either to create micro-organisms of greatly attenuated virulence that may be used as vaccines, or to sequence, and express, potential vaccine antigens. With respect to vaccines composed of purified antigens, parallel work is underway to develop immuno-modulating agents (adjuvants) that will selectively and safely induce the necessary immune response. Finally, within this plethora of vaccine candidates, vaccinologists are devoting much effort to alternatives to immunization via injection, such as administration of a vaccine through the mucosal route (e.g., oral, intranasal, intravaginal, etc.), through the transcutaneous route, and even by expression of vaccine antigens in edible fruits and vegetables.