PrepTest 135, Section 4, Question 9
Birds and mammals can be infected with West(58 more words not shown)
The statements above, if true,(8 more words not shown)
- AWest Nile virus(8 more words not shown)
- BWest Nile virus(15 more words not shown)
- CSome people who(11 more words not shown)
- DWest Nile virus(12 more words not shown)
- EWest Nile virus(10 more words not shown)
Reading the argument
Humans can catch the virus, but they can't pass it back to mosquitoes under the conditions described here. That makes a person a stopping point for this route of transmission.
Work through the two directions carefully. A bird or mammal gets infected only through a mosquito bite. A mosquito can become infected by biting certain infected birds or mammals, but a human's blood never contains enough virus to do that. An infected mosquito can infect a person; biting that person doesn't then infect another mosquito.
So if the virus is spreading in North America, humans can't be the source that keeps that cycle going. Something other than infected people must account for the spread. We don't have to identify that source. And although the passage gives one way mosquitoes become infected, it doesn't say that is their only possible route.
What we’re looking for
For this inference question, I'd look for a consequence of that limit on human transmission.
Working the answers
- A
A disease can be common among humans even if humans don't transmit it onward. Many people could be bitten by infected mosquitoes. The passage limits how the virus spreads; it doesn't set a low limit on how many people can catch it.
- B
More mosquitoes doesn't necessarily mean more of this virus. Imagine one region has many mosquitoes but no infected ones, while another has fewer mosquitoes and an established source of infection. The passage doesn't give us a distribution of infected mosquitoes or justify ranking regions by mosquito density alone.
- C
Not enough virus to infect a mosquito doesn't mean not enough virus to make a person ill. Those are different thresholds. The passage tells us about transmission from human blood, not whether an infected person has symptoms.
- D
Where the virus was first detected doesn't tell us where it infects more people. We have no counts of infections in northern Africa and North America to compare.
- ECorrect
This applies our reading to the virus's arrival, rather than just its continued spread. Suppose an infected person traveled to North America. A mosquito biting that person wouldn't become infected, and the other birds and mammals couldn't acquire the virus directly from that person because they require mosquito bites. The infected traveler therefore couldn't introduce the described chain of transmission. Some other route must account for the virus's spread to North America.
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