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Pandemics have occurred throughout human history,
流行病的發生貫穿人類的歷史，
Some have left scars on the tissue and bone of their victims,
有些病毒在受害者的身體組織和骨頭上留下了傷疤，
while evidence for others comes from preserved DNA.
而有跡象表明這些病毒來自于的古代的DNA。
For instance, scientists have recovered DNA from the bacteria that transmits tuberculosis from the remains of ancient Egyptian mummies.
例如，科學家們從古埃及木乃伊的殘骸中重新找到傳染肺結核的病菌。
And in 2011, scientists investigating a plague pit in the city of London
在2011年，科學家們調查了倫敦城的一處瘟疫區，
were able to reconstruct the genome of Yersinia pestis,
它竟能夠改造鼠疫桿菌基因。
the bacterium responsible for the Black Death of the 14th century.
這些病菌是14世紀黑死病爆發的起因。
It is thought the plague originated in China in around 1340, spreading west along the Silk Road,
人們認為這場瘟疫起源于中國，大概是在1340年，沿著絲綢之路傳播到西方去，
the caravan route running from Mongolia to the Crimea.
商隊的路線從蒙古到克里米亞半島。
In 1347, the plague reached the Mediterranean,
在1347年，這場瘟疫到達了地中海，
and by 1400, it had killed in excess of 34 million Europeans, earning it the title, the Great Mortality.
而直到1400年，這場瘟疫造成超過三千四百萬歐洲人的死亡，使它擁有了“必死”的頭銜。
It was later historians who called it the Black Death.
隨后的歷史學家們稱之為“黑死病”。
However, by far the greatest pandemic killer is influenza.
然而，目前為止最普遍的流行病殺手是流行性感冒。
Flu is constantly circulating between the Southern and Northern Hemispheres.
流感持續不斷地在南北半球循環。
In North America and Europe, seasonal flus occur every autumn and winter.
在北美和歐洲，季節性流感每個秋冬季都會出現。
As the majority of children and adults will have been exposed to the virus in previous seasons, these illnesses are usually mild.
大多數的兒童和成年人在之前都接觸過流感病毒，這些疾病通常是很溫和的。
However, every 20 to 40 years or so the virus undergoes a dramatic mutation.
但是，每20年至40年間，這些流感病毒就會經歷一次急劇變異。
