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.

处女地流行病指的是那些风险人群以前没有接触过袭击他们的疾病，因此在免疫上几乎毫无防御能力的疾病。

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Having long been the most accessible river crossing between Nevada and Arizona, the dam is thought to be at risk of a terror strike, with trucks already banned from crossing.

胡佛大坝长期作为内华达州和亚利桑那州主要的交通连接处，虽然卡车已经禁止通行，但是大坝依然面临恐怖袭击的威胁。

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It also is increasingly used to weed out embryos at risk of genetic diseases — such as breast cancer — that could be treated, or that might not strike a person later in life.

因为存在有某些基因疾病——例如乳腺癌——的风险而被流产的胎儿也越来越多，而这些疾病是可以被治愈的，或者这些疾病也许根本不会发生。

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