Hi, I need some help answering these questions from the Official Guide. I don't understand how they got the answer...confused..

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 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
(10) 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
(15) 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
(20) 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,
(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.

QUESTION:
1. 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


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

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.


3. It can be inferred from the passage that Lyme disease has become prevalent in parts of the United States because of which of the following?

A: The inadvertent introduction of Lyme disease bacteria to the United States
B: The inability of modern sanitation methods to eradicate Lyme disease bacteria
C: A genetic mutation in Lyme disease bacteria that makes them more virulent
D: The spread of Lyme disease bacteria from infected humans to noninfected humans
E: An increase in the number of humans who encounter deer ticks

+1 mne toge intersna eta tema