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. 【S1】