题型:阅读理解 题类:常考题 难易度:普通
河南省濮阳市2019-2020学年高一下学期英语期末试卷
Since December, there have been tens of thousands of the COVID-19 cases both in and out of China, caused by a previously unknown virus: novel coronavirus.
What are they? Viruses are considered to be non-living organisms(有机物).They can't reproduce(繁殖) on their own. Instead, they invaded the cells of living organisms to reproduce, spread and take over. But the basic question is, where did viruses first come from? Until now, no clear explanation for their origin exists.
Understanding the origins of viruses means fully understanding the history of their hosts not only humans, but also bacteria, plants and other animals, which is a complicated task. However, there are three main theories to explain the origin of viruses. First, viruses started as independent organisms, then became parasites(寄生者). Second, viruses evolved(进化) from pieces of DNA or RNA that "escaped" from larger organisms. The third one is that viruses co-evolved with their host cells, which means they existed alongside these cells.
For the time being, these are only theories. The technology and proofs we have today cannot be used to test these theories and discover the most plausible explanation. Continuing studies may provide us with clearer answers. Or future studies may find that the answer is even more mysterious than it now appears.
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