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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
(5) discovered the importance of social and ecological fac-
tors 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
(10) which time polio infection produced paralysis. Previ-
ously, 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
(15) example is Lyme disease, which is caused by bacteria
that are transmitted by deer ticks. It occurred only spo-
radically 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
(20) occurred simultaneously with the growth of the suburbs
and increased outdoor recreational activities in the
deer’s habitat. Similarly, an outbreak of dengue hemor-
rhagic fever became an epidemic in Asia in the 1950’s
because of ecological changes that caused Aedes aegypti,
(25) 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.
选项 A、An outbreak of Lyme disease
B、An outbreak of dengue hemorrhagic fever
C、An epidemic of typhoid
D、An epidemic of paralytic polio among infants
E、An epidemic of paralytic polio among adolescents and adults
答案C
解析 The best answer is C. Line 13 states that modern hygiene practices prevented the spread of typhoid, suggesting that if modern sanitation were discontinued, the likelihood of a typhoid epidemic would increase. Choices A and B can be eliminated because the passage ties Lyme disease and dengue hemorrhagic fever to animal carriers rather than to lack of hygiene. Choice D is incorrect because, according to lines 10-12, polio in infants does not typically cause paralysis. Choice E is incorrect because if modern sanitation is not practiced, polio immunity is acquired during infancy.
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