Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem:Passage: The first case of West Nile virus in North America occurred in 1999. Within just a few years, the virus had spread throughout most of the United States. Birds as well as humans can be infected with the virus. Birds often fly long distances. This is one reason why West Nile virus spread so quickly. Answer: north america
Solution:
Where did west nile first appear?