Risk Virgin Soil Epidemics Populations Previous Contact Diseases Strike

意群训练： Virgin-soil epidemics are those in which the populations at risk have had no previous contact with the diseases that strike them and are therefore immunologically almost defenseless .

标志： Virgin-soil epidemics are those {in which the populations (at risk) have had no previous contact ［with the diseases (that strike them)］and are /therefore/ immunologically almost defenseless }.

译文： 处女地流行病 是 这样一些疾病 ，{在这些疾病中 （危险）人群 以前从 未接触 过［（那些侵袭了他们的）疾病］，而且 /因此/在免疫上几乎是毫无防范的 }。

解释： 在谈到疾病的时候，populations at risk的意思是危险人群，at risk 的英文释义为：exposed to a usually specified danger or loss <patients at risk of infection>。本句修饰those的定语从句是典型的层层修饰。