来自《耶鲁大学开放课程：生物医学工程探索 第10集 第10集》

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  this is the real disease causing agent.
  这是可以直接致病的病毒
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  If you just introduced this polio virus,
  如果你直接接种脊髓灰质炎病毒
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  which you could make in large quantities into people now,
  将大量的病毒直接接种到人体
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  you would be causing polio.
  就会引起小儿麻痹症
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  So, you couldn't introduce the live virus in
  所以 不可以直接接种活体病毒
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  because that would cause the disease not just immunity.
  因为这会使人致病而不仅是引起免疫应答
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  Remember that the lucky thing about the smallpox vaccine was that
  对于天花病毒来说 幸运的是
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  a naturally occurring
  存在一种自然发生的
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  attenuated form of the smallpox virus variola
  被称作痘苗病毒的
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  called vaccinea was found.
  天花病毒的衰减疫苗
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  So, that was a naturally occurring attenuated virus
  因为天花病毒存在天然的衰减病毒
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  that could be produced
  这种衰减病毒可以用来