Until recently, biologists were unable to explain the fact that pathog

Until recently, biologists were unable to explain
the fact that pathogens—disease-causing parasites—
have evolved to incapacitate, and often overwhelm,
their hosts. Such behavior is at odds with the
(5) prevailing view of host-parasite relations—that, in
general, host and parasite ultimately develop a benign
coexistence. This view is based on the idea that
parasites that do not harm their hosts have the best
chance for long-term survival: they thrive because
(10) their hosts thrive. Some biologists, however, recently
have suggested that if a pathogen reproduced so
extensively as to cause its host to become gravely
sick, it could still achieve evolutionary success if its
replication led to a level of transmission into new
(15) hosts that exceeded the loss of pathogens resulting
from the host’s incapacitation. This scenario suggests
that even death-causing pathogens can achieve
evolutionary success.

One implication of this perspective is that a
(20) pathogen’s virulence—its capacity to overcome a
host’s defenses and incapacitate it—is a function of
its mode of transmission. For example, rhinoviruses,
which cause the common cold, require physical
proximity for transmission to occur. If a rhinovirus
(25) reproduces so extensively in a solitary host that the
host is too unwell to leave home for a day, the
thousands of new rhinoviruses produced that day will
die before they can be transmitted. So, because it is
transmitted directly, the common cold is unlikely to
(30) disable its victims.

The opposite can occur when pathogens are
transported by a vector—an organism that can carry
and transmit an infectious agent. If, for example, a
pathogen capable of being transported by a mosquito
(35) reproduces so extensively that its human host is
immobilized, it can still pass along its genes if a
mosquito bites the host and transmits this dose to the
next human it bites. In such circumstances the
virulence is likely to be more severe, because the
(40) pathogen has reproduced to such concentration in the
host that the mosquito obtains a high dose of the
pathogen, increasing the level of transmission to new
hosts.

While medical literature generally supports the
(45) hypothesis that vector-borne pathogens tend to be
more virulent than directly transmitted pathogens—
witness the lethal nature of malaria, yellow fever,
typhus, and sleeping sickness, all carried by biting
insects—a few directly transmitted pathogens such as
(50) diphtheria and tuberculosis bacteria can be just as
lethal. Scientists call these “sit and wait” pathogens,
because they are able to remain alive outside their
hosts until a new host comes along, without relying
on a vector. Indeed, the endurance of these pathogens,
(55) many of which can survive externally for weeks or
months before transmission into a new
host—compared, for instance, to an average
rhinovirus life span of hours—makes them among the
most dangerous of all pathogens.

1. Which one of the following most accurately summarizes the main idea of the passage?

(A) A new hypothesis about the host-incapacitating behavior of some pathogens suggests that directly transmitted pathogens are just as virulent as vector-borne pathogens, due to the former’s ability to survive outside a host for long periods of time.
(B) A new hypothesis about the host-incapacitating behavior of some pathogens suggests that, while most pathogens reproduce so extensively as to cause their hosts to become gravely sick or even to die, some eventually develop a benign coexistence with their hosts.
(C) A new hypothesis about the host-incapacitating behavior of some pathogens suggests that they are able to achieve reproductive success because they reproduce to a high level of concentration in their incapacitated hosts.
(D) A new hypothesis about the host-incapacitating behavior of some pathogens suggests that they are generally able to achieve reproductive success unless their reproduction causes the death of the host.
(E) A new hypothesis about the host-incapacitating behavior of some pathogens suggests that pathogen virulence is generally a function of their mode of transmission, with vector-borne pathogens usually more virulent than directly transmitted pathogens, except for those directly transmitted pathogens able to endure outside their hosts.


2. According to the passage, the prevailing view of the host-parasite relationship is that, in general,

(A) the host is ultimately harmed enough to prevent the parasite from thriving
(B) a thriving parasite will eventually incapacitate its host
(C) a parasite must eventually be transmitted to a new host in order to survive
(D) the parasite eventually thrives with no harm to its host
(E) ultimately the host thrives only if the parasite thrives


3. With which one of the following statements about the prevailing view of host-parasite relations would the biologists mentioned in line 10 be most likely to agree?

(A) The view contradicts most evidence of actual host-parasite relations.
(B) The view suggests that even death-causing pathogens can achieve evolutionary success.
(C) The view presumes the existence of a type of parasite behavior that does not exist.
(D) The view ignores the possibility that there is more than one way to achieve evolutionary success.
(E) The view erroneously assumes that hosts never harm the parasites that feed off them.


4. The examples of diphtheria and tuberculosis bacteria provide the most support for which one of the following conclusions about the dangerousness of pathogens?

(A) The most dangerous pathogens are those with the shortest life spans outside a host.
(B) Those pathogens with the greatest endurance outside a host are among the most dangerous.
(C) Those pathogens transported by vectors are always the most dangerous.
(D) The least dangerous pathogens are among those with the longest life spans outside a host.
(E) Those pathogens transmitted directly are always the least dangerous.


5. Which one of the following, if true, would most seriously challenge the position of the biologists mentioned in line 10?

(A) Most pathogens capable of causing their hosts’ deaths are able to achieve reproductive success.
(B) Most pathogens transmitted from incapacitated hosts into new hosts are unable to overwhelm the new hosts.
(C) Most pathogens that do not incapacitate their hosts are unable to achieve reproductive success.
(D) Most hosts that become gravely sick are infected by pathogens that reproduce to relatively high concentrations.
(E) Most pathogens transmitted from incapacitated hosts are unable to reproduce in their new hosts.


6. Which one of the following most accurately describes the organization of the passage?

(A) introduction of a scientific anomaly; presentation of an explanation for the anomaly; mention of an implication of the explanation; discussion of two examples illustrating the implication; discussion of exceptions to the implication
(B) introduction of a scientific anomaly; presentation of an explanation for the anomaly; discussion of two examples illustrating the explanation; discussion of exceptions to the explanation; mention of an implication of the explanation
(C) introduction of a scientific anomaly; presentation of an explanation for the anomaly; discussion of two examples illustrating the explanation; mention of an implication of the explanation; discussion of examples illustrating the implication
(D) introduction of a scientific anomaly; presentation of an implication of the anomaly; discussion of two examples illustrating the implication; discussion of exceptions to the implication
(E) introduction of a scientific anomaly; discussion of two examples illustrating the anomaly; presentation of an explanation for the anomaly; discussion of examples illustrating the explanation


7. The passage implies that which one of the following is a reason that rhinoviruses are unlikely to be especially virulent?

(A) They immobilize their hosts before they have a chance to reproduce extensively enough to pass directly to new hosts.
(B) They cannot survive outside their hosts long enough to be transmitted from incapacitated hosts to new hosts.
(C) They cannot reproduce in numbers sufficient to allow vectors to obtain high enough doses to pass to new hosts.
(D) They cannot survive long enough in an incapacitated host to be picked up by vectors.
(E) They produce thousands of new rhinoviruses each day.


8. The primary purpose of the passage is to

(A) compare examples challenging the prevailing view of host-parasite relations with examples supporting it
(B) argue that the prevailing view of host-parasite relations is correct but is based on a mistaken rationale
(C) offer a modification to the prevailing view of host-parasite relations
(D) attack evidence that supports the prevailing view of host-parasite relations
(E) examine the origins of the prevailing view of host-parasite relations