Vaccination, or the deliberate induction of protective immunity by administering nonpathogenic forms of a microbe or its antigens to induce a memory immune response, is the world's most cost-effective medical procedure for preventing morbidity and mortality caused by infectious disease. Historically, most vaccines have worked by eliciting long-lived plasma cells. These cells produce antibodies that limit disease by neutralizing a toxin or blocking the spread of the infectious agent. For these 'B cell vaccines,' the immunological marker, or correlate, for protection is the titer of protective antibodies. With the discovery of HIV/AIDS, vaccine development has been confronted by an agent that is not easily blocked by antibody. To overcome this, researchers who are developing HIV/AIDS vaccines have turned to the elicitation of cellular immunity, or 'T cell vaccines,' which recognize and kill infected cells.