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Pandemics have occurred throughout human history,
流行病的发生贯穿人类的历史，
Some have left scars on the tissue and bone of their victims,
有些病毒在受害者的身体组织和骨头上留下了伤疤，
while evidence for others comes from preserved DNA.
而有迹象表明这些病毒来自于的古代的DNA。
For instance, scientists have recovered DNA from the bacteria that transmits tuberculosis from the remains of ancient Egyptian mummies.
例如，科学家们从古埃及木乃伊的残骸中重新找到传染肺结核的病菌。
And in 2011, scientists investigating a plague pit in the city of London
在2011年，科学家们调查了伦敦城的一处瘟疫区，
were able to reconstruct the genome of Yersinia pestis,
它竟能够改造鼠疫杆菌基因。
the bacterium responsible for the Black Death of the 14th century.
这些病菌是14世纪黑死病爆发的起因。
It is thought the plague originated in China in around 1340, spreading west along the Silk Road,
人们认为这场瘟疫起源于中国，大概是在1340年，沿着丝绸之路传播到西方去，
the caravan route running from Mongolia to the Crimea.
商队的路线从蒙古到克里米亚半岛。
In 1347, the plague reached the Mediterranean,
在1347年，这场瘟疫到达了地中海，
and by 1400, it had killed in excess of 34 million Europeans, earning it the title, the Great Mortality.
而直到1400年，这场瘟疫造成超过三千四百万欧洲人的死亡，使它拥有了“必死”的头衔。
It was later historians who called it the Black Death.
随后的历史学家们称之为“黑死病”。
However, by far the greatest pandemic killer is influenza.
然而，目前为止最普遍的流行病杀手是流行性感冒。
Flu is constantly circulating between the Southern and Northern Hemispheres.
流感持续不断地在南北半球循环。
In North America and Europe, seasonal flus occur every autumn and winter.
在北美和欧洲，季节性流感每个秋冬季都会出现。
As the majority of children and adults will have been exposed to the virus in previous seasons, these illnesses are usually mild.
大多数的儿童和成年人在之前都接触过流感病毒，这些疾病通常是很温和的。
However, every 20 to 40 years or so the virus undergoes a dramatic mutation.
但是，每20年至40年间，这些流感病毒就会经历一次急剧变异。
