In the case of the coronavirus that causes Covid-19, countries around the world have adopted movement restrictions on an unprecedented scale and social-distancing measures that together keep people far enough apart so that the virus can’t easily spread.

就新冠病毒而言，世界各国都以前所未有的规模采取了行动限制和保持社交距离的措施——这些措施使人们保持足够远的距离，让病毒无法轻易传播。

3.So how does a virus come back?

病毒如何卷土重来？

There are a number of possibilities. In the case of influenza, there’s the onset of cool weather, a factor that may affect the coronavirus, too. Or the pathogen can mutate. This is another feature of flu, which mutates more or less constantly.

有几种可能性。就流感而言，天气会转凉，这个因素也可能会影响新冠病毒。或者病原体可能会变异。这是流感的另一个特点，基本在不断变异。

In the fall of 1918, a second wave of the historic influenza outbreak occurred and caused most of the deaths in the pandemic. Some researchers believe it was brought about by a mutation that made the virus again unrecognizable to most people’s immune systems. Another important variable is the movement of the virus to populations that haven’t been exposed before and don’t have immunity.

1918年秋天，历史上著名的流感疫情出现第二波暴发，造成了那次大流行中的大多数死亡。有些研究人员认为，那是变异造成的，变异导致大多数人的免疫系统无法识别这种病毒。另一个重要的变化因素是病毒向以前没有接触过它、所以不具备免疫力的人群传播。

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