阅读下面一段文字,完成1-4题。禽流感最早于1878年在意大利被发现,到1955年才知道它是A型流感。在这个过程中,病毒不断变异,也引发了不少疫情。迄今,世界卫生组织确认的禽流感病毒共有15种次类型,令人担忧的是其中能够快速突变的H5N1。医学界对H5N1的广泛关注始于1997年。比较 Read the following paragraph to complete the 1-4 questions. Bird flu was first discovered in Italy in 1878. It was not until 1955 that it was known as influenza A. In the process, the continuous mutation of the virus has also triggered many epidemics. To date, there are 15 subtypes of bird flu virus confirmed by the World Health Organization. Of concern is the rapid mutation of H5N1. The medical community’s extensive attention to H5N1 began in 1997. Comparing