由無害的多種致病介質如病毒或細菌或來自這些介質的蛋白質組成的生物制品。當注入人體時，介質本身或其蛋白質亞基會引發很強的免疫反應從而避免對同種介質的進一步感染。疫苗模仿自然的感染引發強大的免疫反應而不會造成疾病。疫苗中使用的蛋白質通常在致病介質表面找到并可以在實驗室生成。完整的細菌或病毒可以通過熱和輻射處理變得無害，也可以通過生物處理使其活性減弱變成活的無害的介質。這樣的例子包括不能在體溫下生長的流感病毒和在病人體內不能感染神經的小兒麻痹癥疫苗。

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.