由无害的多种致病介质如病毒或细菌或来自这些介质的蛋白质组成的生物制品。当注入人体时，介质本身或其蛋白质亚基会引发很强的免疫反应从而避免对同种介质的进一步感染。疫苗模仿自然的感染引发强大的免疫反应而不会造成疾病。疫苗中使用的蛋白质通常在致病介质表面找到并可以在实验室生成。完整的细菌或病毒可以通过热和辐射处理变得无害，也可以通过生物处理使其活性减弱变成活的无害的介质。这样的例子包括不能在体温下生长的流感病毒和在病人体内不能感染神经的小儿麻痹症疫苗。

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.