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。
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.
其它的都是搭便车的生物，主要是一些与我们共生的无害细菌。
The rest are all hitch-hikers, mainly harmless bacteria that have evolved with us.
但是，如果我们正确理解环境中的生物体是如何在保护我们的话，也许我们可以用疫苗或利用一些无害的刺激来模仿他们的效果。
But if we properly understand how organisms in the environment protect us, maybe we can give a vaccine or mimic their effects with some innocuous stimulus.
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