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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.  
According to the passage, the outbreak of dengue hemorrhagic fever in the 1950s occurred for the reason that______.
选项 A、the mosquito Aedesaegypti was newly introduced into Asia
B、the mosquito Aedesaegypti became more numerous
C、the mosquito Aedesalbopictus became infected with the dengue virus
D、more people began to visit and inhabit areas in which mosquitoes live and breed
答案B
解析 细节题。题目问的是根据文章内容，20世纪50年代登革出血热发生的原因是______。根据“dengue hemorrhagic fever in the 1950s”定位到文中倒数第二句“…because of ecological changes that caused Aedesaegypti，the mosquito that transmits the dengue virus，to proliferate”可知由蚊子的繁殖传播了登革热病毒。故选B。
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