Although genetic mutations in bacteria and viruses can lead to epidemics, so

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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. [br] According to the passage, the outbreak of dengue hemorrhagic fever in the 1950 ’ s occurred for which of the following reasons?

选项 A、The mosquito Aedes aegypti was newly introduced into Asia.
B、The mosquito Aedes aegypti became more numerous.
C、The mosquito Aedes albopictus became infected with the dengue virus.
D、Individuals who would normally acquire immunity to the dengue virus as infants were not infected until later in life.
E、More people began to visit and inhabit areas in which mosquitos live and breed.

答案 B

解析 50年代登革热流行是什么原因?A．埃及伊蚊，文中未提是“newly introduced into Asia”。B．正确。埃及伊蚊突然变多。原文L33—36。C．Aedes albopictus和50年代的登革热无关。D．immunity和登革热无关。E．人们更经常去蚊子聚居区。无。

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