65 LR Two Question 9
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This was really really helpful, but one quick question on this. How do you know that West Nile wasn't carried to North America by a person already infected by a mosquito? My thought was that they may have carried it but not infected other beings (birds, mammals, mosquitoes...).

Well, we are told the virus spread to North America in the third sentence.
Now, you're right that a person could have come to North America while infected with the virus, but she could never have transmitted the virus to anyone (first and last sentences of the passage), thus, she cannot have been responsible for its spread.

We can't prove from the passage people don't transmit the virus, and therefore we can't prove they didn't transport it to Nort America. Maybe they transmit it to reptiles or amphibians, but not to mosquitoes, and therefore not to birds and mammals. We don't know that insects, birds and mammals are the only kind of animals that can have the virus, and we don't know on what species the virus was detected in North America—maybe it was detected only on bullfrogs, and bullfrogs have this peculiarity where they can get the virus from licking human skin (ugh!).
I think this is one of those inference questions where you can't prove the answer choice, but you're given sufficient evidence to take you almost to full proof, and the other answer choices are too far from full proof to be acceptable.