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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  very simple observations.
  这方法基于两个最简单的观察结果
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  The first was that
  第一个就是
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  smallpox was clearly contagious.
  天花病传染性很强
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  The second was that those who had recovered from the disease--
  第二个就是得病痊愈的人
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  and it was easy to know who had suffered from it because they
  谁得过病很容易看出来 因为
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  were scarred and pockmarked, or also they were often blind--
  身上会留疤痕和痘印 而且很多人会变盲
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  and it was a simple observation that they never
  很容易发现这样的人从来
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  caught smallpox a second time.
  不会再患上天花
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  So, the idea developed that it might be a wise measure to
  于是人们就慢慢有了一个想法
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  induce a mild case of smallpox artificially,
  或许以人为方式植入温和化的天花病
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  to protect a person, or especially a child,
  可以保护一个人 尤其是孩子
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  from the risk of acquiring a naturally occurring but severe
  不受自然获得的严重致残致命版