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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 significant 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, infection had occurred during infancy, when it typically provided lifelong immunity without paralysis. Thus, the hygiene that helped prevent typhoid epidemics indirectly fostered a paralytic polio 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 become 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 activities in the deer’s habitat. Similarly, an outbreak of dengue hemorrhagic fever became an epidemic in Asia in the 1950s because of ecological changes that caused Aedesaegypti, 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, Aedesalbopictus.  
It can be inferred from the passage that Lyme disease has become prevalent in parts of the United States because of______.
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、an increase in the number of humans who encounter deer ticks
答案D
解析 细节题。题目问的是根据文章内容，莱姆病在美国部分地区流行的原因是______。根据Lyme disease定位到文中第六、七句“Lyme disease，which is caused by bacteria that are transmitted by deer ticks．．．recently become prevalent in parts of the United States，largely due to an increase in the deer population．．．and increased outdoor recreational activities in the deer’s habitat”可知，莱姆病在美国部分地区流行是由于鹿蜱传播了导致莱姆病的病菌，而越来越多的人又接触到鹿蜱，故选D。
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