Reading Comprehension

PT120 · S2 · P4 · Q24 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.

24.

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

Correct
introduction of a scientific anomaly;
Why this is right
The last ingredient is "exceptions to the implication", which works. The final paragraph discussed some exceptions to the implication that "vector-borne tend to be more virulent than directly transmitted": a few directly transmitted pathogens such as diptheria and tuberculosis bacteria can be just as lethal (as vector-borne pathogens). So let's read the rest: 1. scientific anomaly - "biologists were unable to explain / such behavior is at odds with prevailing view" (beginning of 1st P) 2. presentation of explanation for anomaly - the reason why pathogens often evolve to overwhelm their hosts is that the pathogen can still achieve evolutionary success if it transmits enough into new hosts. (end of 1st P) 3. mention of an implication - "One implication of this perspective is that virulence is connected to mode of transmission" (beginning of 2nd P) 4. discussion of two examples - For example, rhinoviruses are direct transmit / low virulence (2nd paragraph). A pathogen capable of being transported by a mosquito is vector transmit / high virulence (3rd paragraph) 5. discussion of exceptions to the implication - Last paragraph
Skill tested: Organization · how this choice captures the passage's function is the move to repeat next time.
49% picked this
introduction of a scientific anomaly;
Wrong Order
Oddly, this has basically all the same ingredients as (A), just in different order. The last ingredient should go 3rd in this list, since the implication is named at the beginning of paragraph 2. There is no implication mentioned in the last paragraph, just exceptions to the implication.
9% picked this
introduction of a scientific anomaly;
Bad Last Ingredient
When we check in on this last ingredient, it makes it seem like final paragraph was all about examples of an implication. But the last paragraph begins, While medical literature generally supports [the implication], a few directly transmitted pathogens such as diphtheria and tuberculosis bacteria can be just as lethal. And then the rest of the 3rd paragraph goes on to flesh out these examples of exceptions to the implication.
9% picked this
introduction of a scientific anomaly;
Bad Match: implication of anomaly
This is extremely close to (A), so we are pretty much stuck analyzing the difference between (A) and (D). The 2nd ingredient here is "an implication of the anomaly". The 2nd ingredient in (A) is "explanation for the anomaly". The anomaly was that "even though the prevailing view is that host and parasite develop a benign coexistence, there are some anomalous cases where the parasite evolves to overwhelm and incapacitate their hosts". An implication of the anomaly would be something like, "Given that some parasites actually evolve to overwhelm their hosts, we need to figure out which parasites those are so that we can develop medications to limit the parasites' damage." An explanation for the anomaly would be, "How is it that parasites could evolve to overwhelm their hosts?" The second half of the 1st paragraph explains how this anomaly could occur -- even though the parasite overwhelms the host, the parasite could still achieve evolutionary success if its replication led to a level of transmission that exceeded the loss of pathogens from the host being overwhelmed. The implication that gets mentioned at the outset of the 2nd paragraph isn't an implication of the anomaly. It's an implication of "this perspective". What is "this perspective"? It's the perspective of "some biologists" who have suggested a possible explanation for the anomaly.
29% picked this
introduction of a scientific anomaly;
Bad Last Ingredient Wrong Order
When we check in on this last ingredient, it makes it seem like final paragraph was examples that illustrate an explanation, whereas we thought of the last paragraph as discussing nonconforming examples that don't illustrate the main thrust of the passage. Furthermore, the "explanation for the anomaly" comes at the end of the 1st paragraph, not after the two examples are discussed (P2 and P3).
4% picked this

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