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.（1）

A

A

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.（2）

G

G

Sometimes a virus can cause a disease so serious that it is fatal.Other viral infections bring about 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.（3）

E

E

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？（4）

C

C

"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（化石）and because of the tricks they use to make copies of themselves within the cells they've invaded."
   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 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.
   （5）

B

B

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.

A.But what are viruses？
B.For the time being，these are only theories.
C.Until now no clear explanation for their origin exists.
D.The answer may be even less satisfactory than it now appears.
E.Once inside，they try to find a host cell to infect.
F.Experts are concerned about the bad effect that it had on people's health.
G.For this reason，they always have the potential to be dangerous to human life.