Although genetic mutations in bacteria and virusescan lead to epidemics, some epidemics ar

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.

The passage suggests that a lack of modern sanitation would make which of the following most likely to occur？

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