Birds and mammals can be infected with West Nile virus only through

Birds and mammals can be infected with West Nile virus only through mosquito bites. Mosquitoes, in turn, become infected with the virus when they bite certain infected birds or mammals. The virus was originally detected in northern Africa and spread to North America in the 1990s. Humans sometimes catch West Nile virus, but the virus never becomes abundant enough in human blood to infect a mosquito.

Type- inference
1. Mosquitoes, in turn, become infected with the virus when they bite certain infected birds or mammals
2. Humans sometimes catch West Nile virus, but the virus never becomes abundant enough in human blood to infect a mosquito.

(A) West Nile virus will never be a common disease among humans.- incorrect, will never is too strong, we know that humans do not transmit

(B) West Nile virus is most common in those parts of North America with the highest density of mosquitoes.- incorrect, do we know whether birds and mammals are present in that part?

(C) Some people who become infected with West Nile virus never show symptoms of illness.- incorrect, the visibility of symptoms is not discussed

(D) West Nile virus infects more people in northern Africa than it does in North America.- incorrect, we are not given any information to make a comparison

(E) West Nile virus was not carried to North America via an infected person.- Correct- on combining 1 and 2.

Answer E

The threshold value is relevant for infection of other humans through humans not for humans infection.

Answer E as the virus never becomes abundant enough in human blood to infect a mosquito. - If Humans can't infect a mosquito,then in no way could they spread the virus.

nightblade354 - don't you think that this is a very weak CR question? Given that we should answer an inference question with 100% certainty using the information in the prompt, who should one arrive at (E)? Just because humans don't transmit the virus to mosquitoes doesn't mean they cant transmit it human to human.

The statements above, if true, most strongly support which one of the following?

(A) West Nile virus will never be a common disease among humans.

(B) West Nile virus is most common in those parts of North America with the highest density of mosquitoes.

(C) Some people who become infected with West Nile virus never developed symptomsof illness

(D) West Nile virus infects more people in northern Africa than it does in North America.

(E) West Nile virus was not carried to North America via an infected person.

The first sentence of the prompt clearly states that one can be infected "only through mosquito bites" - human-to-human tranmission is not possible.

So the first infected human could have travelled from Africa to NA with the virus, but there is no way for the virus to propagate.

You'll need infected birds + mosquitos in NA - but we dont know which animal went to NA first.

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chrtpmdr, as stated above the fact that it can only infect from mosquito to humans means that this possibility is ruled out.

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The passage can be summarized as follows:

(i) infected Mosquito ----> infects birds and mammals
(ii) Infected birds or mammals ---> infect mosquitoes.
(iii) Humans do catch the virus but so far never became infected with the disease.

The statements above, if true, most strongly support which one of the following?

(A) West Nile virus will never be a common disease among humans. - This is an extreme statement because we do know at so far, the virus has not infected several humans to become common among them. But all that may change if the conditions necessary for proliferation of the virus are present. Hence, we can eliminate (A).

(B) West Nile virus is most common in those parts of North America with the highest density of mosquitoes. - The passage does not make any distinction regarding the 'density' of mosquitoes; the passage only explores the sources (or links) that lead to the spread of the virus. Hence, we can eliminate (B).

(C) Some people who become infected with West Nile virus never show symptoms of illness. - the passage mentions that so far, no one has been infected with the virus. Moreover, the passage does not discuss the symptoms of the illness caused by the virus. Hence, we can eliminate (C).

(D) West Nile virus infects more people in northern Africa than it does in North America. - The passage does not provide us with any information about the 'number of people' infected anywhere. We therefore cannot support (D) with the information provided to us in the passage. Hence, eliminate (D).

(E) West Nile virus was not carried to North America via an infected person. - the passage does mention that ''the virus never becomes abundant enough in human blood to infect a mosquito''. We also know that the mosquito is the carrier of the virus. We can reasonably infer that the virus was not carried to North America via an infected person. Hence, (E) is the right answer choice here.

Thank You

Birds and mammals can be infected with West Nile virus only through mosquito bites. Mosquitoes, in turn, become infected with the virus when they bite certain infected birds or mammals. The virus was originally detected in northern Africa and spread to North America in the 1990s. Humans sometimes catch West Nile virus, but the virus never becomes abundant enough in human blood to infect a mosquito.

The statements above, if true, most strongly support which one of the following?

(A) West Nile virus will never be a common disease among humans. - WRONG. Not necessarily. Two highlighted text are the problems with this option.

(B) West Nile virus is most common in those parts of North America with the highest density of mosquitoes. - WRONG. Like A, this one is also not necessarily true. Four possibilities exist.

(C) Some people who become infected with West Nile virus never show symptoms of illness. - WRONG. Same reasoning.

(D) West Nile virus infects more people in northern Africa than it does in North America. - WRONG. Subjective again, it depends.

(E) West Nile virus was not carried to North America via an infected person. - CORRECT. Had it been carried by human passage breaks apart. Though its an inference question but it looks like an assumption - not an important issue. What is important is how the question stem is formulated?

Answer E.