Vaccines confer protection against specific pathogens by exploiting the specificity and memory of the acquired immune response. The host is first exposed to the pathogen through a preparation of the pathogen's antigens that elicits a primary acquired immune response without developing clinical symptoms of disease in the host. Subsequent exposures to the same antigens elicit faster and more effective secondary immune responses to the offending microbe. Although many vaccines consist of killed bacteria or of live attenuated strains,[17] the antigens in the vaccine need not be associated with whole cells. Recombinant forms of virulence factors, such as fimbriae or heat-labile toxin from enterotoxigenic E. coli, may be sufficient to induce an effective immunologic response.[18]

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