PT120.S2.P4.Q23

PrepTest 120 - Section 2 - Passage 4 - Question 23

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P1

Until recently, biologists were unable to explain the fact that pathogens—disease-causing parasites—have evolved to incapacitate, and often overwhelm, their hosts. ████ ████████ ██ ██ ████ ████ ███ ██████████ ████ ██ █████████████ █████████████████ ██ ████████ ████ ███ ████████ ██████████ ███████ █ ██████ ████████████ ████ ████ ██ █████ ██ ███ ████ ████ █████████ ████ ██ ███ ████ █████ █████ ████ ███ ████ ██████ ███ █████████ █████████ ████ ██████ ███████ █████ █████ ███████ ███

Puzzling Phenomenon · Pathogens incapacitate and kill hosts
This is puzzling because the prevailing view is that parasites need their hosts for long-term survival.
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Hypothesis · Trade-off
Pathogen can cause hosts to be very sick or even die if it can spread to a new host. So it's trading away staying in the current host for jumping to a new host.
P2

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Implication · Virulence is function of mode transmission
Virulence means the ability to defeat the host's defenses and incapacitate the host.
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Example · Close contact transmission requires low virulence
Rhinoviruses (common cold) requires physical contact for transmission. Hence, it cannot be too virulent. If it were, then the host would be confined in solitude (or die) and the virus would not be able to jump to a new host. It needs to be only mildly virulent so that the host can help it jump to new hosts.
P3

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Example · Vector transmission requires high virulence
If a pathogen can be transmitted via a vector (e.g., mosquito), then virulence can and perhaps must be high. It needs to reproduce so extensively (and hence severely harm the host) such that the vector (e.g., mosquito) can acquire enough of the pathogen to transmit it to a new host.
P4

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Anomalies · "Sit and wait" pathogens
In general, the literature supports the hypothesis. But "sit and wait" pathogens like diphtheria and tuberculosis are directly transmitted yet lethal. They can do this because they can survive outside their hosts for up to months. This makes them the most dangerous kind of pathogen.
Passage Style
Phenomenon-hypothesis (RC)
Single position
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23.

Which one of the following, ██ █████ █████ ████ █████████ █████████ ███ ████████ ██ ███ ██████████ █████████ ██ ███ ██████████████ ████████ ██ ███ █████ ██████████

a

Most pathogens capable ██ ███████ █████ ██████ ██████ ███ ████ ██ ███████ ████████████ ████████

This doesn’t weaken the biologists’ view. The biologists would acknowledge (A) and propose that the reason these pathogens can be reproductively successful is because of their rate of reproduction and transmission.

5%
b

Most pathogens transmitted ████ █████████████ █████ ████ ███ █████ ███ ██████ ██ █████████ ███ ███ ██████

This doesn’t weaken the biologists’ view, because the biologists don’t have to believe that pathogens transmitted from incapacitated hosts generally overwhelm the new hosts. As long as pathogens transmitted from incapacitated hosts can continue to survive and reproduce, the biologists’ hypothesis still works.

9%
c

Most pathogens that ██ ███ ████████████ █████ █████ ███ ██████ ██ ███████ ████████████ ████████

This doesn’t weaken the biologists’ view, because this view concerns pathogens that do incapacitate their hosts. What happens with respect to pathogens that don’t incapacitate their hosts doesn’t affect the biologists’ view.

6%
d

Most hosts that ██████ ███████ ████ ███ ████████ ██ █████████ ████ █████████ ██ ██████████ ████ ███████████████

This is consistent with the biologists’ view, because if the hosts that become gravely sick are infected by pathogens that incapacitate their hosts, then the biologists believe these pathogens are likely to reproduce at sufficiently high rates to cause transmission to new hosts to exceed the loss of pathogens from the incapacitated host. A “relatively high concentration” might indicate a sufficiently high rate of reproduction.

3%
e

Most pathogens transmitted ████ █████████████ █████ ███ ██████ ██ █████████ ██ █████ ███ ██████

This weakens the biologists' view, because if most pathogens transmitted from incapacitated hosts can’t reproduce in new hosts, then these pathogens have no way to achieve reproductive and evolutionary success. The biologists’ view assumes that pathogens that incapacitate their host can continue to reproduce when they are transmitted to a new host, and (E) undermines this assumption.

77%

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