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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.
It can be inferred from the passage that Lyme disease has become prevalent in parts of the United States because of which of the following?
选项 A、The inadvertent introduction of Lyme disease bacteria to the United States.
B、The inability of modern sanitation methods to eradicate Lyme disease bacteria.
C、A genetic mutation in Lyme disease bacteria that makes them more virulent.
D、The spread of Lyme disease bacteria from infected humans to noninfected humans.
E、An increase in the number of humans who encounter deer ticks.
答案E
解析 文中指出，为什么莱姆病在美国部分地区流行?此点在原文有明确回答。L26—30。∴E正确，接触鹿蜱的人数增多。A、B、C、D都明显和原文此处叙述不一致。
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