Following the outbreak of the novel coronavirus pneumonia （NCP），also named COVID-19 by WHO， there is a general fear of the unknown virus as its full effects remain to be seen. Fever, coughing, sore throat, difficulty breathing - the NCP's symptoms are similar to the common cold or the flu, but it's potentially more dangerous.
Viruses could be deadly, like HIV and Ebola. But what are viruses？ How can they cause so much trouble？
Viruses are non-living organisms approximately one-millionth of an inch long. Unlike human cells or bacteria, they can't reproduce on their own. Instead, they invade the cells of living organisms to reproduce, spread and take over.
Viruses can infect every living thing - from plants and animals down to the smallest bacteria. For this reason, they always have the potential to be dangerous to human life. Sometimes a virus can cause a disease so serious that it is fatal. Other viral infections trigger（引发） no noticeable reaction.
Viruses lie around our environment all of the time, waiting for a host cell to come along. They can enter our bodies by the nose, mouth, eyes or breaks in the skin. Once inside, they try to find a host cell to infect. For example, HIV， which causes AIDS， attacks the T-cells of the immune system.
But the basic question is, where did viruses first come from？ Until now, no clear explanation for their origin exists. "Tracing the origins of viruses is difficult," Ed Rybicki, a virologist at the University of Cape
Town in South Africa, told Scientific American, "because viruses don't leave fossils and because of the tricks they use to make copies of themselves within the cells they've invaded."
However, there are three main hypotheses （假说） to explain the origin of viruses. First, viruses started as independent organisms, then became parasites （寄生者）． Second, viruses evolved from pieces of DNA or RNA that "escaped" from larger organisms. Third, 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 evidence we have today cannot be used to test these theories and identify the most plausible explanation. Continuing studies may provide us with clearer answers. Or future studies may reveal that the answer is even murkier （含糊不清的） than it now appears.
What can we learn about viruses from the text？ ______
A. Viruses have nothing to do with the common cold.
B. Viruses are really small living organisms.
C. Viruses can't reproduce unless they find a host cell.
D. Viruses enter our bodies mainly through the mouth, nose and hair.
Which of the following might explain the origin of viruses？ ______
A. They evolved from the fossils of large organisms.
B. They evolved from parasites into independent organisms.
C. They evolved from the T-cells in animals.
D. They evolved along with their host cells.
The underlined word "plausible" in the last paragraph probably means ______ ．
A. reasonable
B. common
C. creative
D. unbelievable
What can we conclude from the text？ ______
A. Viruses live longer in human host cells than in animals'．
B. Viruses will become more like bacteria as they evolve.
C. It may take a long time to understand the origin of viruses.
D. The author is optimistic about future virus research.
