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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  among the general population.
  爆发为主要流行病
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  A couple of factors came into play, to reinforce this pattern.
  一些因素加强了这种模式运作
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  Obviously not every child contracted the disease,
  显然 不是每个儿童都会染上这种疾病
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  and so over time
  因而随着时间的推移
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  there'd be a slow accumulation of non-immune,
  不免疫的 易感染的
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  susceptible adults who could fall ill of the disease.
  易得病的成人越来越多
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  It was also true that European cities in the early modern era
  早期现代欧洲城市的运作
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  were so unhealthy
  都非常不健康
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  that they sustained or expanded their population,
  以至于他们得靠大量外来人口涌入
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  not by growth from within,
  而不是自身人口增长
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  but by a constant influx of people from without.
  来保持及扩大人口数量
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  Peasants driven off the land,
  农民也许由于饥饿或战乱