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  3. Although genetic mutations in bacteria and viruses can lead to epidemics, some epidemics are caused by bacteria and viruses that

Although genetic mutations in bacteria and viruses can lead to epidemics, some epidemics are caused by bacteria and viruses that

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问题     Although genetic mutations in bacteria and viruses can lead to epidemics, some epidemics are caused by bacteria and viruses that have undergone no sig- nificant genetic change. In analyzing the latter, scientists have discovered the importance of social and ecological factors to epidemics. Poliomyelitis, for example, emerged as an epidemic in the United States in the twentieth century; by then, modern sanitation was able to delay exposure to polio until adolescence or adulthood, at which time polio infection produced paralysis. Previously, in- fection had occurred during infancy, when it typically provided lifelong immunity without paralysis. Thus, the hygiene that helped prevent typhoid epidemics indirectly fostered a paralytic po- lio epidemic. Another example is Lyme disease, which is caused by bacteria that are transmitted by deer ticks. It occurred only sporadically during the late nineteenth century but has recently be- come prevalent in parts of the United States, largely due to an increase in the deer population that occurred simultaneously with the growth of the suburbs and increased outdoor recreational activ- ities in the deer’s habitat. Similarly, an outbreak of dengue hemorrhagic fever became an epidemic in Asia in the 1950’s because of ecological changes that caused Aedes aegypti, the mosquito that transmits the dengue virus, to proliferate. The stage is now set in the United States for a dengue epidemic because of the inadvertent introduction and wide dissemination of another mosquito, Aedes albopictus.
Which of the following, if true, would most strengthen the author’s assertion about the cause of the Lyme disease outbreak in the United States?
选项 A、The deer population was smaller in the late nineteenth century than in the mid-twentieth century.
B、Interest in outdoor recreation began to grow in the late nineteenth century.
C、In recent years the suburbs have stopped growing.
D、Outdoor recreation enthusiasts routinely take measures to protect themselves against Lyme disease.
E、Scientists have not yet developed a vaccine that can prevent Lyme disease.
答案A
解析 哪个会支持作者关于莱姆病流行原因的解释?原文解释：由于两个原因：鹿种群数增加。人们更多地去鹿栖息地活动。∴A正确，鹿群在19世纪晚期比20世纪中期少。B、C、D、E皆不符原文解释。
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