来自《耶鲁大学开放课程：1600年后西方社会的流行病 第7集 第7集》

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  and most feared killer of its time.
  人们最为恐惧的杀手
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  Now, in its history, the fact that a sufferer--
  观其历史
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  and we mentioned this last time--
  我们上次曾提过
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  who recovered possessed a robust,
  天花患者痊愈后获得了健全的永久免疫力
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  lifelong immunity to this disease was important.
  这点是十分重要的
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  No one was naturally infected twice with smallpox.
  没有人自然而然感染两次天花
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  So, a typical pattern emerged in the cities of Europe,
  因而 天花成了一种常见疾病
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  and that was that smallpox became an ever-present disease
  安然度过童年的人们也都饱受它的折磨
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  that most people who survived childhood had suffered.
  这成为了欧洲城市的典型模式
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  The adult population therefore possessed what we might call
  因此 成年人群拥有了我们所谓的
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  an extensive herd immunity to smallpox as a disease.
  对天花的彻底群体免疫力
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  So, it became an endemic disease of childhood.
  因而 天花演变为童年时期的地方病