Passage Summary: West Nile virus moves back and forth between mosquitoes and animals, but even though humans can get sick from it, we are a 'dead end' because we can't pass it back to the mosquitoes.

Reasoning: West Nile virus cycles between mosquitoes and certain animals; while humans can be infected, the virus does not reach high enough levels in human blood to be passed back to mosquitoes.

Analysis: This set of facts establishes a clear biological boundary for the virus's transmission. Because the virus in human blood is never 'abundant enough' to infect a mosquito, the chain of transmission stops at the human. Look for an answer that synthesizes this: if a mosquito cannot get the virus from a human, then a human cannot be the source of infection for another person via a mosquito. The logic here is airtight—humans are essentially a dead-end street for this particular virus.