biological preparations composed either of a harmless variety of a disease-causing agent such as a virus or bacteria, or of proteins derived from such an agent. when injected into humans, the agent itself or its protein subunits, will elicit a strong immune response, which will be protective against further infection from that agent. the vaccine “mimics” a natural infection to elicit a strong immune response, but causes no disease. the proteins used in vaccines are usually found on the surface of the disease-causing agents and can be generated in the laboratory. intact bacteria or viruses can be rendered harmless by heat- or radiation- mediated killing, or can be “attenuated” (inactivated) by biomanipulation to produce a live but harmless version of the agent. examples of such “attenuated” or “live vaccines” include influenza virus that does not grow at body temperature and polio vaccine in which the virus cannot infect neurons but remains in the gut of the patient.