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  3. Avian influenza (禽流感), or "bird flu", is an infectious disease of animals was caused by viruses that normally infect

Avian influenza (禽流感), or "bird flu", is an infectious disease of animals was caused by viruses that normally infect

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问题     Avian influenza (禽流感), or "bird flu", is an infectious
disease of animals was caused by viruses that normally infect             【S1】______
only birds and, less commonly, pigs. Avian influenza viruses are
highly species-specific, but have, on rare occasions, crossed the
species barrier to infecting humans.                                      【S2】______
    In domestic poultry (家禽), infection with avian influenza
viruses cause two main forms of disease, distinguished by                 【S3】______
low and high extremes of virulence (毒性). The so-called
"low pathogenic (病原的)" form commonly causes only mild
symptoms and may easily go undetected. The highly pathogenic
form is far more dramatic. It spreads very rapidly through
poultry flocks, causes disease affecting multiple external organs,         【S4】______
and has a mortality that can approach 100%, often within 48
hours.
    Bird flu is not the same as SARS. Because their symptoms               【S5】______
are similar, SARS is caused by completely different viruses.
Influenza viruses also are more infectious and cannot be as
readily contained as SARS isolating people who have the                    【S6】______
infection.
    The current outbreak of bird flu is different from earlier
ones in that officials have been unable to contain its spread. An
outbreak in 1997 in Hong Kong was the first time the virus had
spread to people, but it was much more quickly contained. A
total of 18 people were hospitalized with six reporting deaths.            【S7】______
About 1.5 million chickens were killed by an effort to remove              【S8】______
the source of the virus.
    Unlike the 1997 scare, this outbreak has spread more rapidly
to other countries, increasing its exposure on people in varied            【S9】______
locations and raised the likelihood that the strain will combine           【S10】______
with a human influenza virus.
【S4】
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