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 9rovided 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 albopictus．
Questions6-11 refer to the passage above.
6. 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
7. 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 mosquitoes live and breed．
8. 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
9. 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．
10． Which of the following best describes the organization of the passage?
(A) A paradox is stated，discussed，and left unresolved．
(B) Two opposing explanations a re presented，argued，and reconciled. (C) A theory is proposed and is then followed by descriptions of three experiments that support the theory．
(D) A generalization 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．
11． Which of the following l 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 enthusiasts 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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Homeostasis，an animal's maintenance of certain internal variables within an acceptable range，particularly in extreme physical environments，has long interested biologists．The desert rat and the camel in the most water-deprived environments，and marine vertebrates in an all-water environment，encounter the same regulatory problem：maintaining adequate Internal fluid balance． For desert rats and camels，the problem is conservation of water in an environment where standing water is nonexistent，temperature is high, and humidity is low．Despite these handicaps, desert rats are able to maintain the osmotic pressure of their blood，as well as their total body water content，at approximately the same levels as other rats．one countermeasure is behavioral：these rats stay in burrows during „the hot part of the day, thus avoiding loss of fluid through panting or sweating，which are regulatory mechanisms for maintaining internal body temperature by evaporative cooling. Also，desert rats‟ kidneys can excrete a urine having twice as high a salt content as sea water．
Camels，on the other hand，rely more on simple endurance．They cannot store water, and their reliance on an entirely unexceptional kidney results in a rate of water loss through renal function significantly higher than that of desert rats．As a result，camels must tolerate losses In Body water of up to thirty percent of their body weight．Nevertheless，camels do rely on a special mechanism to keep water loss within a tolerable range：by sweating and panting only when their body temperature exceeds that which would kill a human，they conserve internal water． Marine vertebrates experience difficulty with their water balance because though there is no shortage of seawater to drink, they must drink a lot of it to maintain their internal fluid balance．But the excess salts from the seawater must be discharged somehow，and the kidneys of most marine vertebrates are unable to excrete a urine in which the salts are more concentrated than in seawater．Most of these animals have special salt-secreting organs outside the kidney that enable them to eliminate excess salt.
Questions 12-15 refer to the passage above.
12． Which of the following most accurately states the purpose of the passage?
(A) To compare two different approaches to the study of homeostasis (B) To summarize the findings of several studies regarding organisms maintenance of internal variables in extreme environments
(C) To argue for a particular hypothesis regarding various organisms conservation of water in desert environments
(D) To cite examples of how homeostasis is achieved by various organisms
(E) To defend a new theory regarding the maintenance of adequate fluid balance
13． According to the passage，the camel maintains internal fluid balance in which of the following ways?
I． By behavioral avoidance of exposure to conditions that lead to fluid loss
II． By an ability to tolerate high body temperatures
III． By reliance on stored internal fluid supplies
(A) I only
(B) II only
(C) I and II only
(D) II and III only
(E) I,II , and Ill
14．It can be inferred from the passage that some mechanisms that regulate internal body temperature t like sweating and panting, can lead to which of the following?
(A) A rise in the external body temperature
(B) A drop in the body's internal fluid level
(C) A decrease i n the osmotic pressure of the blood
(D) A decrease in the amount of renal water loss
(E) A decrease in the urine's salt content
15．It can be inferred from the passage that the author characterizes the camel's kidney as＂entirely unexceptional＂(1ine 27)primarily to emphasize that it
(A) functions much as the kidney of a rat functions
(B) does not aid the camel in coping with the exceptional water loss resulting from the extreme conditions of its environment
(C) does not enable the camel to excrete as much salt as do the kidneys of marine vertebrates
(D) is similar in structure to the kidneys of most mammals living in water-deprived environments
(E) requires the help of other organs in eliminating excess salt
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