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At times, it is actually possible to watch the effects of natural selection in host-parasite relationships.

有时，在宿主-寄生虫关系中观察自然选择的影响实际上是可能的。

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Thus, natural selection stabilized this host-parasite relationship.

因此，自然选择让这种寄主-寄生关系变得稳定。

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The only way that we knew how to grow it reliably was to grow it in its natural host: cows.

那目前我们所知道的唯一途径就是,利用它的自然宿主,奶牛

耶鲁公开课 - 生物医学工程探索课程节选

But no one knows which disease will be next, what animal will be its natural host, where it will happen, or how dangerous it will be.

ECONOMIST: Bird flu and public health

Until researchers can determine what animal is the natural host of the virus, and how MERS spreads from the host to humans, each new outbreak is dangerous and mysterious.

CNN: Why MERS virus is so scary

Placing unaccompanied children and orphans with host families proved a natural solution.

CNN: 'Mama Africa' and Congo's orphans of war