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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 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
Reading Passage "Genetic Mutations " Duplicated Here

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