Reading Comprehension

PT120 · S2 · P4 · Q19 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.

19.

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

  1. A new hypothesis about the

    Too Strong: just as virulent Contradicted

    The main clause here is that the New idea is that "directly transmitted are just as virulent as vector-borne". Yes, some directly transmitted (such as diphtheria and tuberculosis) are just as lethal / virulent. But the whole reason the author is writing this passage is to inform us of this general relationship they've now realized is going on in which "vector-borne tend to be more virulent than directly transmitted".

    10% picked this

  2. A new hypothesis about the

    Contradicted: most vs. some Wrong Emphasis

    The main clause of this answer is that there's a new hypothesis which says "some pathogens eventually develop a benign coexistence with their hosts". That's not the New idea. The benign coexistence is the Old, prevailing view. We'd rather hear this answer say, "while most pathogens develop a benign coexistence with their hosts, some pathogens may achieve evolutionary success even while reproducing so extensively as to cause their hosts to become gravely sick or even die". The main focus of the passage was the more-virulent pathogens, which are the exceptions not the usual. This answer also fails to mention the main takeaway, which is that there's a strong connection between mode of transmission and virulence.

    1% picked this

  3. A new hypothesis about the

    Wrong Justification

    This says that the big new takeaway is that some super virulent pathogens are able to still have reproductive success because they reproduce to a high level of concentration in their hosts. That is part of the story, but not enough. On its own, reproducing to a high level in the host just overwhelms, incapacitates, and potentially kills the host. The secret sauce of how this can be a successful evolutionary strategy is the idea of the vector (or in rare cases, the sit-and-wait capacity). The true measure of reproductive success is that "replication leads to a level of transmission into new hosts that exceeds the loss of pathogens resulting from the host's incapacitation."

    12% picked this

  4. A new hypothesis about the

    Opposite

    This makes the New idea out to be, "some pathogens are able to achieve reproductive success unless their reproduction causes the death of the host". The main point was really, "some pathogens are able to achieve reproductive success even if their reproduction causes the death of the host", as long as they're being transmitted by a vector and so the level of transmission into new hosts exceeds the loss of pathogens resulting from the old host's death.

    1% picked this

  5. Correct

    A new hypothesis about the

    Why this is right

    According to the main clause of this answer, the New idea is that "pathogen virulence is generally a function of mode of transmission, with vector usually more virulent than direct". That looks like a really strong match for our Author's Take on the New, at the beginning of the final paragraph. This answer even adds a little qualifier idea at the end to wrap its arms around the exceptional cases of mean & nasty directly transmitted pathogens alluded to in the final paragraph.

    Skill tested: Main Point · how this choice captures the passage's function is the move to repeat next time.

    76% picked this

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