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  3. An epidemic of swine flu (猪流感) has recently developed in Mexico and the United States, says the CDC. Swine flu has killed many p

An epidemic of swine flu (猪流感) has recently developed in Mexico and the United States, says the CDC. Swine flu has killed many p

admin2011-04-04  34
问题     An epidemic of swine flu (猪流感) has recently developed in Mexico and the United States, says the CDC. Swine flu has killed many people, and the outbreak has features that suggest it could become a global pandemic (瘟疫). A pandemic is an epidemic that spreads around the whole world. Pandemics also often cause more severe disease than epidemics.
    Flu is a disease caused by the influenza virus. Humans, pigs, birds and other animals all can be infected by influenza viruses. Typically, influenza viruses can infect only one species, so the influenza viruses of humans are different from those of pigs and birds. However, sometimes a virus can infect more than one species. For examples, pigs sometimes can be infected not only with pig influenza viruses, but also with human and bird influenza viruses. Then these viruses can come up to one another secretly and swap (交换) genes, creating new viruses that have a mix of genes -- from human, pig, and bird viruses. That is what has happened with this new swine flu virus.
    Sometimes this swapping of genes allows a virus that was originally able to infect only pigs or only birds to also infect humans. When that happens, we refer to the illness as "swine flu" or "bird flu". This current virus could actually be called "swine/bird flu", since it has some genes from pig flu viruses and other genes from bird flu viruses.
    Most viruses that cause swine flu or bird flu are very hard to pass from one human to another: they don’t cause epidemics. Sometimes, however, further changes in genes create a virus that can spread rapidly among humans, and can produce a more severe illness. One reason this illness is more severe is that the virus is so new. The regular flu that comes each year is caused by a regular human influenza virus that often has similarities to the viruses that have caused the flu in years past, so people have some degree of immunity to the latest virus. The unusual swine flu or bird flu viruses that develop the ability for person-to-person spread are so different that people have little or no immunity to them.
    The worst global pandemic in modem times was the influenza pandemic of 1918 to 1919. It affected about a third of the human race, and killed at least 40 million people in less than a year -- more than those who have been killed by AIDS in three decades. The world economy went into a deep recession. The average length of life dropped for 10 years.
    Unfortunately, the new swine flu virus can be transmitted between humans. It is not clear yet how easily it is transmitted, nor how it is transmitted. Almost surely it is transmitted by sneezing and coughing, and by skin-to-skin contact with an infected person.
The current swine flu threatens the humans more severely than previous flus because ______.
选项 A、it has killed more people
B、it has been found in more countries
C、people know nothing about it
D、people have no immunity to it
答案D
解析 根据题干中的more severely than previous flus，将本题出处定位在对普通流感和猪流感进行比较的第四段。第三句说，这次的流感更严重的一个原因在于病毒太新了，以至于出现了本段最后一句中的情况：人们对它们没有免疫力(people have little or no immunity to them)，这种否定可以理解为，人类对这种新病毒最没有免疫力。而人们对年年来袭的普通流感多多少少都有一定程度的免疫力。所以[D]项的说法正确，符合原因的可递推性。
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