Although genetic¹ mutations in bacteria and viruses can lead to epidemics⁴, some epidemics are caused by bacteria and viruses that have undergone no significant genetic change. In analyzing⁵ the latter, scientists have (5) discovered the importance of social and ecological⁶ fac- tors to epidemics. Poliomyelitis, for example, emerged as an epidemic³ in the United States in the twentieth century; by then, modern sanitation⁷ was able to delay exposure to polio until adolescence⁸ or adulthood⁹, at (10) which time polio infection produced paralysis¹⁰. Previ- ously, infection had occurred during infancy¹¹, when it typically provided lifelong immunity¹² without paralysis. Thus, the hygiene¹³ that helped prevent typhoid epidemics indirectly¹⁴ fostered a paralytic¹⁵ polio epidemic. Another (15) example is Lyme disease, which is caused by bacteria that are transmitted by deer ticks. It occurred only spo- radically¹⁶ 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 simultaneously¹⁷ 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 proliferate¹⁸ The stage is now set in the United States for a dengue epidemic because of the inadvertent introduction and wide dissemination¹⁹ of another mosquito, Aedes albopictus.

　　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 eradicate²⁰ Lyme disease bacteria

　　(C) A genetic mutation² in Lyme disease bacteria that makes them more virulent²¹

　　(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

　　4. Which of the following can most reasonably be concluded about the mosquito Aedes albopictus on the basis of information given in the passage?

　　(A) It is native to the United States.

　　(B) It can proliferate only in Asia.

　　(C) It transmits the dengue virus.

　　(D) It caused an epidemic of dengue hemorrhagic fever in the 1950's.

　　(E) It replaced Aedes aegypti in Asia when ecological changes altered Aedes aegypti's habitat.

　　5. Which of the following best describes the organization of the passage?

　　(A) A paradox²² is stated, discussed and left unresolved.

　　(B) Two opposing explanations are presented, argued, and reconciled.

　　(C) A theory is proposed and is then followed by descriptions of three experiments that support the theory.

　　(D) A generalization²³ is stated and is then followed by three instances that support the generalization.

　　(E) An argument is described and is then followed by three counterexamples that refute the argument.

　　6. Which of the following, if true, would most strengthen the author's assertion about the cause of the Lyme disease outbreak in the United States?

　　(A) The deer population was smaller in the late nineteenth century than in the mid-twentieth century.

　　(B) Interest in outdoor recreation began to grow in the late nineteenth century.

　　(C) In recent years the suburbs have stopped growing.

　　(D) Outdoor recreation enthusiasts²⁴ routinely take measures to protect themselves against Lyme disease.

　　(E) Scientists have not yet developed a vaccine that can prevent Lyme disease

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