Reading Comprehension

PT120 · S2 · P4 · Q26 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.

26.

The primary purpose of the passage is to

  1. compare examples challenging the prevailing

    Wrong Emphasis: comparison

    This is fairly tempting since the author does cite examples challenging the prevailing view (malaria, yellow fever, typhus, and sleeping sickness) and does cite examples supporting it (rhinoviruses like the common cold). This answer wouldn't have anything to say about the final paragraph, where author shows examples that neither fit the prevailing view nor fit the revised "virulence is a function of mode of transmission" view. So this answer is less comprehensive than the correct answer. It also just misses the purpose to say the author merely wanted to compare these examples with those examples. The #1 reason the author sat down to write this is to talk about the new view: "virulence is a function of mode of transmission: direct will normally be milder and vector will normally be more severe".

    28% picked this

  2. argue that the prevailing view

    Out of Scope: mistaken rationale

    The author's final paragraph shows that there are some exceptions to the new view that "vector-borne pathogens tend to be more virulent than directly transmitted pathogens". But that final paragraph isn't saying that the new view is wrong and therefore that prevailing view was essentially correct but based on a mistaken rationale. The prevailing view was that parasites won't evolve to incapacitate their hosts, but both the 3rd and 4th paragraph are talking about parasites that do incapacitate / kill their hosts. The prevailing 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 their hosts thrive". Our author never tries to argue that there's anything mistaken about that. She's just presenting the new perspective that, "even if that's the best chance for long-term survival, there are other evolutionary strategies that can still work, such as replicating enough that the transmission into new hosts exceeds the loss of pathogens resulting from the host being incapacitated."

    3% picked this

  3. Correct

    offer a modification to the

    Why this is right

    Many of us may not be in love with modification of the prevailing view, but it seems to be the best available option. If we were looking for some Old / New language like, "to present this new perspective on host-parasite relations", then we can at least be pleased that "offering a modification" can match up with "discussing this new view". The prevailing view was that "in general, host and parasite ultimately develop a benign coexistence". The modification being offered recently by some biologists focuses on those atypical cases in which the parasite incapacitates the host. It adds this concept that if the mode of transmission is vector based, then the parasite could evolve towards greater virulence, so that the vector gets a nice dose of the disease to take to the next host.

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

    64% picked this

  4. attack evidence that supports the

    Out of Scope: attack evidence

    The evidence for the prevailing view was the concept that "parasites that don't harm their hosts have the best chance for long-term survival" and that mild diseases like rhinoviruses demonstrate how a parasite 'wants' its host to not be harmed, to still go to work so that the rhinovirus can spread to new hosts. The passage isn't trying to attack any of those ideas. Rather, it's finding a way to deal with the fact that there seem to be exceptions to that rule. There seem to be parasites that do have evolutionary success, even while causing death. So the passage is attempting to explain how that's possible and to help us appreciate this overall correlation between mode of transmission and virulence, which we're just now appreciating.

    2% picked this

  5. examine the origins of the

    Wrong Emphasis: the origins

    While the first half of the 1st paragraph gives us some of the underpinnings of the prevailing view, the passage then moves onto talking about this New view, and does so for the rest of the passage. This passage is primarily about the New view, not the prevailing view.

    3% picked this

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