来自《耶鲁大学开放课程：生物医学工程探索 第9集 第9集》

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  humans are the only organisms that are affected by it.
  人类是唯一受天花感染的生物
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  That's important, right?
  这很重要 对吧
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  Because if it was a disease that was carried by
  因为假如人和松鼠都能携带
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  both humans and squirrels, for example,
  某种疾病的话
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  then in order to get rid of it on the planet,
  想要把这种疾病从地球上清除
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  you'd have to vaccinate not only all the people
  不仅要给所有人接种疫苗
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  but all the squirrels, right?
  还要给所有松鼠接种疫苗
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  Otherwise you could have a squirrel host--
  否则松鼠就会变成宿主
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  I'm using squirrels,
  我拿松鼠举例
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  because I think that's funnier than other animals to think of—
  是因为我觉得它比别的动物更有趣
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  but it could be any potential host, right?
  但潜在宿主可以是任何动物 对吧
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  And it would be really difficult to vaccinate a wildlife.
  而给野生动物接种是非常困难的