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》

• 
  
  lifelong immunity to this disease was important.
  这点是十分重要的
• 
  
  No one was naturally infected twice with smallpox.
  没有人自然而然感染两次天花
• 
  
  So, a typical pattern emerged in the cities of Europe,
  因而 天花成了一种常见疾病
• 
  
  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
• 
  
  that most people who survived childhood had suffered.
  这成为了欧洲城市的典型模式
• 
  
  The adult population therefore possessed what we might call
  因此 成年人群拥有了我们所谓的
• 
  
  an extensive herd immunity to smallpox as a disease.
  对天花的彻底群体免疫力
• 
  
  So, it became an endemic disease of childhood.
  因而 天花演变为童年时期的地方病
• 
  
  But at intervals,
  然而 在一般人群中
• 
  
  perhaps every generation or so,
  每隔一代或几代人
• 
  
  smallpox would erupt as a major epidemic
  天花也会时不时地
• 
  
  among the general population.
  爆发为主要流行病