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 9rovided 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 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.


According to the passage, the outbreak of dengue hemorrhagic fever in the 1950's occurred for which of the following reasons?


The mosquito Aedes aegypti was newly introduced into Asia.

The mosquito Aedes aegypti became more numerous.

The mosquito Aedes albopictus became infected with the dengue virus.

Individuals who would normally acquire immunity to the dengue virus as infants were not infected until later in life.

More people began to visit and inhabit areas in which mosquitoes live and breed.

考题讲解

此讲解的内容由AI生成,还未经人工审阅,仅供参考。

正确答案是 B。根据文章,1950年代亚洲的登革热流行是由于生态因素引起的,这些因素使Aedes aegypti蚊子大量繁衍繁殖。而 A,C,D 和 E 都不是引起登革热流行的因素,所以不是正确答案。

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