Reading Comprehension

PT120 · S2 · P4 · Q21 Pathogens

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

Topic

Why some pathogens are unusually virulent (harmful to hosts), and how their transmission method explains this.

Framework

Answer / Explain

Main Point

The main point is that a pathogen's mode of transmission helps explain its level of virulence; specifically, pathogens that are transmitted by vectors or can survive long outside a host tend to be more virulent because they can spread even after disabling their host. **Most Valuable Sentence:** (Paragraph 2, first sentence)

P1: The Puzzle of Pathogen Virulence

Why do some pathogens harm or kill their hosts when you’d expect them to try not to? The classic view is that parasites thrive best when their hosts thrive, but recent biologists challenge this, showing that a pathogen might still win (evolutionarily speaking) if it spreads fast enough to new hosts—even if it makes current hosts very sick.

P2: Transmission Mode as the Key Factor

How contagious a pathogen is depends on how it spreads. For example, common cold viruses need victims to be up and about to infect others, so they can't afford to debilitate their hosts too much—otherwise, they die with the bedridden person.

P3: The Vector-Borne Exception

When pathogens hitch a ride on another organism (like mosquitoes), all bets are off. They can make the host really sick, even immobilize them, and still spread, because their vector does all the work. The sicker the host, the more virus there is for the mosquito to pick up and transport to new victims.

21.

With which one of the following statements about the prevailing view of host-parasite relations would the biologists mentioned in the second-to-last sentence of the first paragraph be most likely to agree?

The view contradicts most evidence
Too Strong: most
The biologists are pointing out that it's possible for a parasite to have a non-benign relationship to its host and still achieve evolutionary success. They aren't claiming that more than 50% of actual host-parasite relations fall into this category. They might freely admit that 80% of parasites develop a benign coexistence. They're just saying it's not a given that all parasites will evolve towards benign coexistence.
3% picked this
The view suggests that even
Opposite
The biologists would say that, "The prevailing view fails to consider that even death-causing pathogens can achieve evolutionary success."
32% picked this
The view presumes the existence
Too Strong: does not exist
This answer, like (A), makes it sound like the biologists are denying the idea that parasites and hosts often have benign coexistence. The prevailing view presumes the existence of benign parasite / host relationships. The biologists aren't arguing, "Nope. Such a thing does not exist." They're arguing. "Yes, but in addition there are also cases of non-benign parasite / host relationships."
1% picked this
Correct
The view ignores the possibility
Why this is right
The biologists would say, "the prevailing view that a parasite achieves evolutionary success best by developing a benign relationship with the host where the host thrives ignores the possibility that a parasite could achieve evolutionary success alternatively by replicating in new hosts, even if it kills its hosts, as long as the rate of replication exceeded the rate of killing hosts." Thus, they'd say that the prevailing view is ignoring the possibility that parasites can still achieve evolutionary success even if they do not develop a benign coexistence with hosts, even if the host does not thrive.
Skill tested: Non-Author Opinion · how this choice captures the passage's function is the move to repeat next time.
63% picked this
The view erroneously assumes that
Out of Scope: harming parasite
The biologists would say, "This view erroneously assumes that parasites ultimately evolve so that they don't harm the host they're feeding off." The biologists, nor the passage, ever talk about the idea of the host harming the parasite.
1% picked this

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