It is a mystery that has puzzled doctors for decades. But experts believe they have now finally solved the tricky question of why winter is notoriously known as the flu season.
According to a new research, the influenza virus coats itself in a protective fatty shield that is tough enough to resist cold temperatures. The butter-like material only melts when it hits the respiratory tract（呼吸道）, leaving the virus free to infect cells-- a process scientists say is like an M&M melting in the mouth.
However, in warmer outdoor temperatures, the protective coating melts before it reaches a person or an animal, killing the bacteria before they can infect someone. Joshua Zimmerberg, from the National Institute of Child Health and Human Development (NICHD), in the US, which led the study, said: “Like an M&M in your mouth, the protective covering melts when it enters the respiratory tract. It's only in this liquid period that the virus is able to enter a cell to infect it.”
In the past, scientists worked on theories that flu is more common in winter because people spend more time inside or the radiation from the sun in summer kills off germs. But no research successfully explained the spread of the disease.
Researchers have already claimed that the report, which is published in the journal Nature Chemical Biology, could lead to new ways to prevent and treat flu. “Now that we understand how the flu virus protects itself so that it can spread from person to person, we can work on ways to prevent it.”
Influenza and other respiratory viruses are spread in small drops broadcast by coughing, sneezing and talking and which can also settle onto surfaces, to be picked up on fingertips.
1. What can be the best title of the passage?
A. The mystery that has puzzled doctors for decades.    B. New ways to prevent and treat flu.
C. A discovery of how flu strikes in Winter.            D. Ways found to prevent flu.
2. Which of the following statements is NOT true?
A. Doctors have been trying hard to discover why flu strikes in winter.
B. In the past, scientists didn’t know how flu spreads.
C. It is possible that flu virus will no longer threaten humans in winter.
D. People get affected by flu because they spend much more time inside in winter.
3. Where should you insert the following paragraph into the passage?
Duane Alexander, the director of NICHD, said: “The study results open new avenues of research for thwarting (使…受挫)winter flu outbreaks.”
A. Between Paragraphs 1 and 2.     B. Between Paragraphs 3 and 4. 
C. Between Paragraphs 4 and 5.     D. Between the last two paragraphs.
4.In what process or order does flu occur?
a. The virus infects cells                 b. The influenza virus was born
c. The butter-like material melts           d. The virus hits respiratory tract
e. The virus coats itself in butter-like shield  f. The virus is free
A. a, c, d, e, b, f .     B. c, f, d, b, e, a.    C. b, e, d, c, f, a.    D. d, b, e, c, f, a.

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