Imagine if half the people in your neighborhood, your city, or even your whole country were wiped out.
想象一下，如果你居住的街区、城市，甚至你的国家，一半人口突然死亡。
It might sound like something out of an apocalyptic horror film,
这听起来像是末日启示录类型的恐怖电影，
but it actually happened in the 14th century during a disease outbreak known as the Black Death.
但这是在14世纪真实发生的事情，当时爆发了一种叫做黑死病的疾病。
Spreading from China through Asia, the Middle East, Africa and Europe,
从中国蔓延到亚洲、中东、非洲和欧洲，
the devastating epidemic destroyed as much as 1/5 of the world's population, killing nearly 50% of Europeans in just four years.
这场毁灭性的传染病夺走了全世界1/5人口的生命，四年时间里，50%欧洲人丧生。
One of the most fascinating and puzzling things abut the Black Death
有关黑死病，最神奇、最令人费解的问题在于
is that the illness itself was not a new phenomenon but one that has affected humans for centuries.
它并不是一个新病种，而是已经困扰了人类几个世纪的疾病。
DNA analysis of bone and tooth samples from this period,
对这个时代人类遗留的骸骨和牙齿样本的DNA分析，
as well as an earlier epidemic known as the Plague of Justinian in 541 CE,
以及更早期于公元541年爆发的查士丁尼大瘟疫，
has revealed that both were caused by Yersinia pestis, the same bacterium that causes bubonic plague today.
分析结果显示两种疾病都是由鼠疫耶尔辛杆菌引起的，同样的细菌导致了今天的黑死病。
What this means is that the same disease caused by the same pathogen can behave and spread very differently throughout history.
这意味着，同样的病原体引发的同样疾病，在历史上会展示出非常不同的症状，并以不同的方式传播。
Even before the use of antibiotics, the deadliest oubreaks in modern times,
即使在抗生素出现之前，现代社会最致命的疾病爆发，
such as the ones that occurred in early 20th century India, killed no more than 3% of the population.
比如二十世纪初在印度爆发的疾病导致的死亡人数，亦不到人口总数的3%。
Modern instances of plague also tend to remain localized, or travel slowly, as they are spread by rodent fleas.
而且现代社会中的传染病传播范围比较有限，速度也比较慢，就像多毛蚤传播的那些疾病。
But the medieval Black Death, which spread like wildfire, was most likely communicated directly from one person to another.
但是中世纪的黑死病像野火一样蔓延，似乎是直接由一个人传染给另外一个人。
And because genetic comparisons of ancient to modern strains of Yersinia pestis
将古代和现代的耶尔辛杆菌的基因做比对，
have not revealed any significantly functional genetic differences,
并没有发现明显的基因差别，
the key to why the earlier outbreak was so much deadlier must lie not in the parasite but the host.
那么早期传染病死亡率更高的主要原因必然不是寄生病菌，而是宿主。
For about 300 years during the High Middle Ages,
在中世纪的300年里，
a warmer climate and agricultural improvements had led to explosive population growth throughout Europe.
温暖的气候和农业的进步，让欧洲人口大幅度增长。
But with so many new mouths to feed, the end of this warm period spelled disaster.
但是随着人口增加，温暖气候的结束带来了灾难。

High fertility rates combined with reduced harvest,
高人口出生率，加上农业减产，
meant the land could no longer support its population, while the abundant supply of labor kept wages low.
意味着土地无法养活这些人口，而且劳动力供给过剩导致收入偏低。
As a result, most Europeans in the early 14th century experienced a steady decline in living standards,
所以14世纪初的大部分欧洲人都经历了一个生活品质逐渐下降的过程，
marked by famine, poverty and poor health, leaving them vulnerable to infection.
其中包括饥荒、贫困、健康状况下降，导致他们易受传染病伤害。
And indeed, the skeletal remains of Black Death victims found in London show telltale signs of malnutrition and prior illness.
实际上，伦敦黑死病死亡者遗留的骸骨中都有营养不良和先天疾病的迹象。
The destruction caused by the Black Death changed humanity in two important ways.
黑死病所造成的危害从两个重要方面改变了人类历史进程。
On a societal level, the rapid loss of population led to important changes in Europe's economic conditions.
从社会层面来说，人口的迅速减少导致欧洲经济结构的重大变化。
With more food to go around, as well as more land and better pay for the surviving farmers and workers,
食物更多了，有更多的土地，幸存下来的农民和工人拿到了更好的报酬，
people began to eat better and live longer as studies of London cemeteries have shown.
对伦敦墓地的调查显示，人们吃得更好，寿命更长了。
Higher living standards also brought an increase in social mobility,
更好的生活条件还推高了社会流动性，
weakening feudalism, and eventually leading to political reforms.
削弱了封建势力，最终带来了政治改革。
But the plague also had an important biological impact.
瘟疫还造成了重大的生物学影响。
The sudden death of so many of the most frail and vulnerable people
大部分体弱的人口突然死亡，
left behind a population with a significantly different gene pool,
留下了一个具有完全不同基因库的人群，
including genes that may have helped survivors resist the disease.
其中就包括了能够帮助幸存者对抗疾病的基因。
And because such mutations often confer immunities to multiple pathogens that work in similar ways,
因为这种突变经常代表对多种类似病原体的免疫力，
research to discover the genetic consequences of the Black Death has the potential to be hugely beneficial.
所以研究黑死病所带来的基因变化具有重要的意义。
Today, the threat of an epidemic on the scale of the Black Death has been largely eliminated thanks to antibiotics.
今天，黑死病蔓延的威胁已经被基本消除了，这要感谢抗生素。
But the bubonic plague continues to kill a few thousand people worldwide every year,
但是淋巴腺鼠疫每年依然在导致数千人丧生，
and the recent emergence of a drug-resistant strain threatens the return of darker times.
而且近期出现的耐药菌株似乎有让我们回到黑暗时代的迹象。
Learning more about the causes and effects of the Black Death is important,
了解黑死病的病因和影响非常重要，
not just for understanding how our world has been shaped by the past.
不仅仅是要了解我们的世界在过去发生了怎样的变化。
It may also help save us from a similar nightmare in the future.
它还可能帮助我们避免梦魇在未来重现。